Add Swift conversion parity

[olilarkin]

Summary

Adds the first Swift parity slice for the C++/WASM API by expanding CLibrosa and Librosa around numeric conversion helpers, frequency helpers, octave/tuning helpers, and frequency weighting.

Details

• Adds C ABI exports for frame/sample/time conversions, vector pitch/mel conversions, octave/tuning helpers, CQT and tempo frequency helpers, and frequency weighting.
• Adds Swift wrappers for the new C ABI surface.
• Extends Swift tests to cover the new parity APIs against the same formulas used by C++/WASM.
• Adds a macOS swift test CI job so Swift package coverage is enforced on PRs.

Validation

• swift test

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