Add Clippy CI gate

[rmorse]

Summary

• Add Clippy to the macOS CI workflow with -D warnings gates for workspace, async, and Metal builds.
• Clean the current warning baseline across tests, examples, benches, xtask, and the sys build script.
• Update contributing and publishing docs so the documented checks match CI.

Notes

• The lint setup stays conservative: default Clippy lints plus -D warnings, not clippy::pedantic.
• The old compile_fail_tests cfg stub was removed because it was commented-out dead code. If we want compile-fail coverage later, we should add real trybuild tests.

Local verification

• git diff --check
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo clippy -p whisper-cpp-plus --all-targets --features async -- -D warnings
• cargo test --workspace -- --test-threads=1
• cargo test -p whisper-cpp-plus --features async -- --test-threads=1

Metal Clippy is wired into macOS CI and will be verified there.