Add proc audio preprocessing contract regression tests

[LuisMRaimundo]

Adds a Phase 12 proc_audio preprocessing contract layer.

New file:
tests/phase_12/test_proc_audio_preprocessing_contract_additional.py

Adds 22 helper-level regression tests covering:

• FFT/STFT helper contracts
• synthetic signal edge cases
• descriptor routing
• density metric vector routing
• frequency-floor guards
• temporal evolution on synthetic STFT data
• determinism and non-mutation

Validation:

• 22 passed for the new file
• 1171 passed, 3 skipped for the full suite
• proc_audio.py coverage: 65.1% → 66.1%
• overall scoped coverage: 66.5% → 66.9%

Production code untouched.