Add a precision parameter to proxy round reads

[Fletch153]

Summary

• latest_round / get_round take a precision argument; the proxy downscales the stored answer to it.
• The cache is untouched — one feed, one ID, one stored scale; callers choose precision at read time.

Stacks on #161. Supersedes the canonical-ID approach explored in #173.

Changes

• Proxy reader: precision: u32 on latest_round and get_round; precision > stored → UnsupportedDecimals (51); a nonzero answer truncating to zero → AnswerTruncatedToZero (52); decimals/description unchanged pass-throughs.
• New mod scaling tests: stored-precision identity, 8dp/6dp downscale, get_round scaling, negative truncation toward zero, both error paths; end-to-end downscale against the real cache.
• Proxy self-upgrade fixture rebuilt for the new signature.

Testing

• cargo test in contracts/data-feeds — 223 pass (180 cache unchanged, 9 common, 34 proxy).

Notes

• The data ID presented must be the exact configured ID (cache keys by raw ID); byte 7 is ignored by the proxy — precision comes only from the parameter.
• Bindings on feature/DF-25434/data-feeds-deployment (Add data feeds cache and proxy bindings #162) need regenerating after this lands.

[github-actions]

Soroban Contract Test Coverage

92.64% line coverage — 18001 / 19431 lines hit

Metric	Hit	Total	Coverage
Lines	18001	19431	92.64%
Functions	1189	1529	77.76%
Regions	33476	35839	93.41%

Per-Contract Breakdown

Contract	Lines (Hit / Total)	Line Cov	Funcs (Hit / Total)	Func Cov
other	18001 / 19431	92.6%	1189 / 1529	77.8%

Full file-level coverage report

Filename                                                 Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ccip-ramp-registry/src/lib.rs                                188                75    60.11%           8                 3    62.50%         104                41    60.58%           0                 0         -
ccip-ramp-registry/src/test.rs                               214                 0   100.00%           5                 0   100.00%         128                 0   100.00%           0                 0         -
ccip-ramp-registry/src/types.rs                               83                11    86.75%          17                 6    64.71%          83                11    86.75%           0                 0         -
ccvs/committee-verifier/src/lib.rs                           588                54    90.82%          26                 2    92.31%         382                10    97.38%           0                 0         -
ccvs/committee-verifier/src/test.rs                         1300                25    98.08%          48                 2    95.83%         587                13    97.79%           0                 0         -
ccvs/committee-verifier/src/types.rs                          17                 8    52.94%           6                 5    16.67%          14                 7    50.00%           0                 0         -
ccvs/versioned-verifier-resolver/src/lib.rs                  287                21    92.68%          14                 1    92.86%         162                 4    97.53%           0                 0         -
ccvs/versioned-verifier-resolver/src/test.rs                 680                 9    98.68%          20                 0   100.00%         359                 3    99.16%           0                 0         -
ccvs/versioned-verifier-resolver/src/types.rs                 89                 6    93.26%          16                 4    75.00%          83                 4    95.18%           0                 0         -
common/authorization/src/allowlist.rs                        167                51    69.46%          12                 4    66.67%         110                40    63.64%           0                 0         -
common/authorization/src/lib.rs                              426                27    93.66%          16                 0   100.00%         277                16    94.22%           0                 0         -
common/authorization/src/ownable.rs                          102                13    87.25%           9                 1    88.89%          62                 6    90.32%           0                 0         -
common/authorization/src/test.rs                            1195                 0   100.00%          57                 0   100.00%         462                 0   100.00%           0                 0         -
common/guard/src/initializable.rs                             30                 1    96.67%           4                 0   100.00%          22                 1    95.45%           0                 0         -
common/guard/src/lib.rs                                       32                 1    96.88%           4                 0   100.00%          22                 0   100.00%           0                 0         -
common/guard/src/test.rs                                     101                 0   100.00%           8                 0   100.00%          47                 0   100.00%           0                 0         -
common/helpers/src/curse_checkable.rs                        103                24    76.70%           6                 1    83.33%          56                12    78.57%           0                 0         -
common/helpers/src/map_updater.rs                             94                15    84.04%           7                 3    57.14%          39                 8    79.49%           0                 0         -
common/helpers/src/soroban_invoke.rs                         204                 4    98.04%           8                 0   100.00%          81                 0   100.00%           0                 0         -
common/interfaces/src/ccip_receiver.rs                         8                 8     0.00%           8                 8     0.00%           8                 8     0.00%           0                 0         -
common/interfaces/src/committee_verifier.rs                   13                13     0.00%          13                13     0.00%          13                13     0.00%           0                 0         -
common/interfaces/src/fee_quoter.rs                           17                17     0.00%          17                17     0.00%          17                17     0.00%           0                 0         -
common/interfaces/src/offramp.rs                              11                11     0.00%          11                11     0.00%          11                11     0.00%           0                 0         -
common/interfaces/src/onramp.rs                                9                 9     0.00%           9                 9     0.00%           9                 9     0.00%           0                 0         -
common/interfaces/src/ramp_registry.rs                         5                 5     0.00%           5                 5     0.00%           5                 5     0.00%           0                 0         -
common/interfaces/src/rmn_proxy.rs                             6                 6     0.00%           6                 6     0.00%           6                 6     0.00%           0                 0         -
common/interfaces/src/router.rs                                9                 9     0.00%           9                 9     0.00%           9                 9     0.00%           0                 0         -
common/interfaces/src/siloed_lock_release_pool.rs             21                21     0.00%          21                21     0.00%          21                21     0.00%           0                 0         -
common/interfaces/src/token_admin_registry.rs                  8                 8     0.00%           8                 8     0.00%           8                 8     0.00%           0                 0         -
common/interfaces/src/token_lock_box.rs                        6                 6     0.00%           6                 6     0.00%           6                 6     0.00%           0                 0         -
common/interfaces/src/token_pool.rs                           11                11     0.00%          11                11     0.00%          11                11     0.00%           0                 0         -
common/interfaces/src/versioned_verifier_resolver.rs          10                10     0.00%          10                10     0.00%          10                10     0.00%           0                 0         -
common/message/src/lib.rs                                    586                41    93.00%          28                 4    85.71%         298                 9    96.98%           0                 0         -
common/message/src/test.rs                                   641                 2    99.69%          28                 0   100.00%         323                 0   100.00%           0                 0         -
common/pool/src/decimals.rs                                  109                10    90.83%           6                 0   100.00%          76                 3    96.05%           0                 0         -
common/pool/src/finality_codec.rs                             52                 1    98.08%           3                 0   100.00%          41                 0   100.00%           0                 0         -
common/pool/src/lib.rs                                       460                31    93.26%          32                 1    96.88%         409                26    93.64%           0                 0         -
common/pool/src/rate_limit.rs                                180                 7    96.11%           9                 0   100.00%         145                 3    97.93%           0                 0         -
common/pool/src/types.rs                                      20                14    30.00%          16                14    12.50%          30                14    53.33%           0                 0         -
common/signature/src/config.rs                                44                12    72.73%           6                 2    66.67%          28                 5    82.14%           0                 0         -
common/signature/src/quorum.rs                                68                 1    98.53%           2                 0   100.00%          44                 1    97.73%           0                 0         -
common/signature/src/scheme.rs                                45                 0   100.00%           2                 0   100.00%          29                 0   100.00%           0                 0         -
cre/forwarder/src/error.rs                                     8                 3    62.50%           1                 0   100.00%           8                 3    62.50%           0                 0         -
cre/forwarder/src/lib.rs                                     460                24    94.78%          18                 0   100.00%         313                 7    97.76%           0                 0         -
cre/forwarder/src/test.rs                                   2840                14    99.51%         125                 2    98.40%        1176                 8    99.32%           0                 0         -
cre/forwarder/src/types.rs                                    62                 6    90.32%          10                 6    40.00%          36                 6    83.33%           0                 0         -
cre/test/read_fixture/src/lib.rs                             125               125     0.00%          33                33     0.00%         103               103     0.00%           0                 0         -
cre/test/receiver/src/lib.rs                                  30                30     0.00%           4                 4     0.00%          15                15     0.00%           0                 0         -
fee-quoter/src/lib.rs                                        771               164    78.73%          20                 3    85.00%         527               100    81.02%           0                 0         -
fee-quoter/src/test.rs                                       861                 0   100.00%          17                 0   100.00%         427                 0   100.00%           0                 0         -
fee-quoter/src/types.rs                                       13                13     0.00%          13                13     0.00%          13                13     0.00%           0                 0         -
mcms/src/abi_encoding.rs                                     216                 4    98.15%          10                 0   100.00%          98                 1    98.98%           0                 0         -
mcms/src/constants.rs                                         10                 0   100.00%           2                 0   100.00%           6                 0   100.00%           0                 0         -
mcms/src/crypto.rs                                           202                 3    98.51%           7                 0   100.00%         109                 2    98.17%           0                 0         -
mcms/src/error.rs                                              8                 4    50.00%           1                 0   100.00%           8                 4    50.00%           0                 0         -
mcms/src/lib.rs                                              713                50    92.99%          20                 0   100.00%         483                23    95.24%           0                 0         -
mcms/src/test.rs                                            2278                19    99.17%          59                 0   100.00%        1095                 2    99.82%           0                 0         -
mcms/src/types.rs                                             10                10     0.00%          10                10     0.00%          10                10     0.00%           0                 0         -
offramp/src/lib.rs                                           729               400    45.13%          23                 8    65.22%         473               224    52.64%           0                 0         -
offramp/src/test.rs                                          890                 0   100.00%          28                 0   100.00%         363                 0   100.00%           0                 0         -
offramp/src/types.rs                                          21                 9    57.14%           6                 5    16.67%          17                 7    58.82%           0                 0         -
onramp/src/lib.rs                                           1181               149    87.38%          27                 2    92.59%         739                61    91.75%           0                 0         -
onramp/src/test.rs                                          1591                 4    99.75%          37                 0   100.00%         708                 4    99.44%           0                 0         -
onramp/src/types.rs                                           23                 9    60.87%           6                 5    16.67%          19                 7    63.16%           0                 0         -
pools/burn-mint-pool/src/lib.rs                              390               107    72.56%          28                 9    67.86%         282                55    80.50%           0                 0         -
pools/burn-mint-pool/src/test.rs                            2671                24    99.10%          56                 5    91.07%        1744                55    96.85%           0                 0         -
pools/lock-release-pool/src/lib.rs                           404               100    75.25%          28                 8    71.43%         288                48    83.33%           0                 0         -
pools/lock-release-pool/src/test.rs                         2330                24    98.97%          55                 5    90.91%        1524                55    96.39%           0                 0         -
pools/siloed-lock-release-pool/src/lib.rs                    556               196    64.75%          38                17    55.26%         384               123    67.97%           0                 0         -
pools/siloed-lock-release-pool/src/test.rs                  1303                12    99.08%          38                 2    94.74%         695                18    97.41%           0                 0         -
pools/token-lock-box/src/lib.rs                              197                17    91.37%          13                 1    92.31%         113                 2    98.23%           0                 0         -
pools/token-lock-box/src/test.rs                             257                 0   100.00%           7                 0   100.00%          73                 0   100.00%           0                 0         -
registry/src/lib.rs                                            8                 0   100.00%           1                 0   100.00%           3                 0   100.00%           0                 0         -
registry/src/test.rs                                          27                 0   100.00%           1                 0   100.00%          11                 0   100.00%           0                 0         -
rmn_proxy/src/lib.rs                                          66                12    81.82%           5                 1    80.00%          34                 3    91.18%           0                 0         -
rmn_proxy/src/test.rs                                        279                 0   100.00%          11                 0   100.00%          91                 0   100.00%           0                 0         -
rmn_remote/src/lib.rs                                        302                26    91.39%          16                 1    93.75%         171                10    94.15%           0                 0         -
rmn_remote/src/test.rs                                       393                 0   100.00%          14                 0   100.00%         133                 0   100.00%           0                 0         -
router/src/lib.rs                                            593                87    85.33%          18                 1    94.44%         374                43    88.50%           0                 0         -
router/src/test.rs                                          1186                 0   100.00%          29                 0   100.00%         497                 0   100.00%           0                 0         -
router/src/test_panic_receiver.rs                              5                 0   100.00%           2                 0   100.00%           5                 0   100.00%           0                 0         -
router/src/types.rs                                            3                 3     0.00%           3                 3     0.00%           3                 3     0.00%           0                 0         -
timelock/src/error.rs                                          6                 2    66.67%           1                 0   100.00%           6                 2    66.67%           0                 0         -
timelock/src/lib.rs                                          649                46    92.91%          32                 2    93.75%         393                18    95.42%           0                 0         -
timelock/src/roles.rs                                        141                10    92.91%           6                 0   100.00%          76                 7    90.79%           0                 0         -
timelock/src/storage.rs                                      115                15    86.96%          12                 1    91.67%          68                 7    89.71%           0                 0         -
timelock/src/test.rs                                        1479                16    98.92%          37                 0   100.00%         531                 1    99.81%           0                 0         -
timelock/src/types.rs                                          3                 3     0.00%           3                 3     0.00%           3                 3     0.00%           0                 0         -
token-admin-registry/src/lib.rs                              368                23    93.75%          18                 1    94.44%         257                 4    98.44%           0                 0         -
token-admin-registry/src/test.rs                             825                 0   100.00%          21                 0   100.00%         287                 0   100.00%           0                 0         -
token-admin-registry/src/types.rs                              2                 2     0.00%           2                 2     0.00%           2                 2     0.00%           0                 0         -
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                                                      35839              2363    93.41%        1529               340    77.76%       19431              1430    92.64%           0                 0         -```

</details>

> Commit `d7294028` | [Full CI run](https://github.com/smartcontractkit/chainlink-stellar/actions/runs/31219332668)

<!-- Sticky Pull Request Commentcontract-coverage -->