Merge develop: Clippy gate and promotion CI optimization

[rmorse]

Summary

• Adds Clippy checks to macOS CI for workspace, async, and Metal targets.
• Cleans up the existing warning baseline across tests, examples, benches, and helpers.
• Documents the Clippy gate in the contribution and publishing docs.
• Skips the heavy macOS test job for direct develop -> main promotion PRs, relying on the already-green develop push run while still running full CI on pushes to develop and main.

Verification

• develop CI rerun passed before the promotion optimization: https://github.com/operator-kit/whisper-cpp-plus-rs/actions/runs/27468516516
• Previous Clippy PR CI passed: https://github.com/operator-kit/whisper-cpp-plus-rs/actions/runs/27468015410

Merge strategy: normal merge commit into main, preserving the develop integration point.