ci(rust): fail the build on rustdoc and rustc warnings

[hubcio]

Two classes of warning were accumulating unseen. The doctest
leg ran cargo doc with no gate and stayed green through 69
rustdoc diagnostics across 20 crates; broken intra-doc links
are the costly ones, since they render as plain text and the
docs quietly lose their navigation. Separately, the clippy
leg denies warnings but runs with --all-features while the
test legs build at default features, so a warning that only
shows up when a feature is off was invisible - that is how
four dead_code findings in the integration test binary sat
in CI output.

RUSTDOCFLAGS closes the first and is cache-safe: it enters
the fingerprint of doc units only, never lib or bin units.

For the second the Cargo lints table beat RUSTFLAGS=-D
warnings. RUSTFLAGS enters the fingerprint of every unit
including registry dependencies, so it forces a full rebuild
of the dep tree and would have to be mirrored into
coverage-baseline, the sole writer of the shared cargo
cache. Measured both ways here: the lints table rebuilt 50
workspace crates and zero registry crates.

Eleven crates keep a local [lints.rust] block because cargo
refuses to merge an inherited table with the [lints.clippy]
they already carry. That makes their nursery = "warn"
effectively deny, since rustc resolves the warnings group at
emission time and catches anything left at warn whatever the
flag order. CI behaviour is unchanged: the clippy leg
already appended -D warnings.

Most rustdoc fixes drop the link brackets where the target
is private and rustdoc has nothing to point at under
--no-deps. Widening visibility to satisfy a doc comment is
backwards, and --document-private-items would publish the
internals of every crate.

The dead ci-qemu feature goes with it; nothing has
referenced it since the monorepo restructure in 8db64db.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.65%. Comparing base (2b1406b) to head (f87cfb3).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3925      +/-   ##
============================================
- Coverage     83.87%   83.65%   -0.22%     
  Complexity     1358     1358              
============================================
  Files          1212     1212              
  Lines        166845   166845              
  Branches     134304   134432     +128     
============================================
- Hits         139935   139568     -367     
- Misses        23269    23489     +220     
- Partials       3641     3788     +147     

Components	Coverage Δ
Rust Core	84.62% <ø> (-0.03%)	⬇️
Java SDK	66.67% <ø> (ø)
C# SDK	75.00% <ø> (-1.48%)	⬇️
Python SDK	90.13% <ø> (ø)
PHP SDK	84.48% <ø> (ø)
Node SDK	94.03% <ø> (-1.92%)	⬇️
Go SDK	68.32% <ø> (ø)

Files with missing lines	Coverage Δ
core/cli/src/args/context.rs	100.00% <ø> (ø)
core/common/src/types/options/mod.rs	86.66% <ø> (ø)
core/common/src/wire_conversions.rs	90.89% <ø> (ø)
core/configs/src/server_config/cluster.rs	98.70% <ø> (ø)
core/configs/src/server_config/server.rs	85.48% <ø> (ø)
core/configs_derive/src/lib.rs	100.00% <ø> (ø)
...igs/connectors/http_provider/response_extractor.rs	87.62% <ø> (ø)
core/connectors/sinks/http_sink/src/lib.rs	85.70% <ø> (-0.09%)	⬇️
core/consensus/src/client_table.rs	88.84% <ø> (ø)
core/consensus/src/dvc_merge.rs	93.61% <ø> (ø)
... and 25 more

... and 61 files with indirect coverage changes

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