feat: Exponential Lock + Conviction System

[unarbos]

Summary

• Implements an exponential lock mechanism where coldkeys can lock alpha stake to a specific hotkey per subnet, building conviction over time.
• Lock state uses lazy evaluation: locked_mass decays as exp(-dt/tau) * X and conviction grows as the time-integral of locked mass via exp(-dt/tau) * (Y + dt * X).
• Enforces lock invariant on unstake (validate_remove_stake) and move/transfer (validate_stake_transition): total coldkey alpha on subnet must remain >= current locked amount.
• New extrinsic lock_stake at call_index(134) with TauBlocks configurable decay timescale (default ~30 days).
• 52 tests covering: green-path operations, incremental top-ups, decay math, invariant enforcement, multi-subnet locks, conviction aggregation, subnet king queries, lock cleanup, swap interactions, and edge cases.

Known gaps documented by tests

The following issues are documented by failing-forward tests that prove the bypass exists:

1. do_recycle_alpha / do_burn_alpha bypass lock — calls decrease_stake_for_hotkey_and_coldkey_on_subnet without lock check.
2. clear_small_nomination_if_required bypasses lock — removes nominator alpha without lock validation.
3. Subnet dissolution orphans Lock entries — destroy_alpha_in_out_stakes removes all Alpha but not Lock storage; stale locks persist across netuid reuse.
4. Coldkey swap does not migrate Lock entries.
5. Hotkey swap does not update Lock entries.

Test plan

• 52 tests in tests/locks.rs — all pass
• Full pallet suite: 1003 passed, 0 failed, 7 ignored

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