Gate the wide-observable test helpers on their users' features and lint pecos-decoders with default features in CI

[ciaranra]

Summary

cargo clippy -p pecos-decoders --all-targets -- -D warnings with default features has been red on dev: three helpers in the spec::build test module (width_dem, assert_wide_correct, assert_64_then_65_rejected) are only called from tests gated on optional decoder features, so a default-feature build sees dead code. Workspace-wide clippy never showed it because feature unification enables those features, and CI's per-crate default-feature lane did not include this crate.

Changes

• The three helpers now carry the union of their users' feature gates (pymatching, ldpc, relay-bp for the wide-observable helper; tesseract, fusion-blossom, uf, mwpf for the 64-then-65 helper; all seven for the shared DEM builder), with a comment explaining why.
• pecos-decoders is added to the "clippy without LLVM features" per-crate lane in rust-test.yml, so a default-feature regression in this crate fails CI instead of hiding behind workspace feature unification.

Verification

• cargo clippy --locked -p pecos-decoders --all-targets -- -D warnings: clean for default features, --all-features, and each of the eight features alone (ldpc, fusion-blossom, mwpf, pymatching, tesseract, chromobius, relay-bp, uf), and via the exact CI-lane invocation from inside the crate directory.
• cargo test --locked -p pecos-decoders: 29 pass (default), 46 pass (--all-features).
• A default-feature clippy sweep over the other feature-bearing crates (pecos-foreign, pecos-quantum, pecos-qec, pecos-phir, pecos-neo, pecos-lindblad, pecos-zx, pecos-wasm, pecos-simulators, pecos-core, pecos-qasm, pecos-phir-json, pecos) found no other instance.
• pre-commit run --all-files: pass.