fix: improve ReplayGain analysis accuracy and robustness

[M-Igashi]

Summary

This PR addresses three issues related to ReplayGain analysis and MP4 metadata handling:

Issue #41: Limited sample rate support

• Before: Only 3 sample rates (48000, 44100, 32000 Hz) were supported, with all others falling back to 44100 Hz coefficients
• After: All 12 sample rates from the original mp3gain are now supported (96000, 88200, 64000, 48000, 44100, 32000, 24000, 22050, 16000, 12000, 11025, 8000 Hz)
• Unsupported sample rates now return an error instead of silently using incorrect coefficients

Issue #43: Missing denormal float prevention

• Added 1e-10 constant to filter processing to prevent denormal float slowdowns
• This matches the original gain_analysis.c implementation
• Improves performance when processing audio with silent sections

Issue #47: Bounds checking for MP4 metadata parsing

• Added checked_add and saturating_add for safer arithmetic
• Prevents potential panics when parsing malformed MP4 files

Testing

• All existing tests pass
• Added comprehensive test for all 12 supported sample rates
• Added test to verify unsupported sample rates are properly rejected

References

• Filter coefficients from: https://github.com/cpuimage/ReplayGainAnalysis/blob/master/gain_analysis.c
• Related issue in headroom: Panic on Non-Standard Characters in Filenames results in failed processing headroom#7

Fixes #41
Fixes #43
Fixes #47