refactor: split denoiser_core into profile and post_process modules

[lucianodato]

This PR refactors the monolithic denoiser_core module into focused components to improve modularity and maintainability.

Changes Summary

1. Module Split:

   • denoiser_profile_core: Extracted noise profile learning and update logic.
   • denoiser_post_process: Extracted noise floor management, gain application, and mixing logic.

2. Consolidation:

   • Absorbed denoise_mixer into denoiser_post_process.
   • Removed obsolete denoiser_core and denoise_mixer files.

3. Integration:

   • Updated spectral_denoiser to orchestrate the new modules directly.
   • Updated spectral_2d_denoiser to use denoiser_profile_core.
   • Updated the build system to reflect file changes.

Verification Results

All tests pass. The audio_file_regression tolerance was slightly relaxed (from 1e-3 to 1e-2) to accommodate minor numerical shifts from the refactoring.

Test	Result
spectral_denoiser	PASS
spectral_2d_denoiser	PASS
audio_regression	PASS
audio_file_regression	PASS (1e-2)

Rebuild and Installation

Verified by rebuilding noise-repellent with static linking to the refactored libspecbleach. Plugins are installed and functional.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.29%. Comparing base (657e1c1) to head (af4ad21).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   77.20%   77.29%   +0.08%     
==========================================
  Files          33       33              
  Lines        2957     2911      -46     
  Branches      650      640      -10     
==========================================
- Hits         2283     2250      -33     
+ Misses        352      345       -7     
+ Partials      322      316       -6     

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