fix: handle last frame before APE/ID3 tags and M4A info display

[M-Igashi]

Summary

This PR fixes two issues:

Issue #54: Last frame not modified when followed by APE/ID3 tag

Problem: When an MP3 file has metadata tags (APE or ID3v1) immediately following the last audio frame, mp3rgain was skipping the last frame during gain application.

Solution: Added find_audio_end() function that detects trailing APE/ID3v1 tags and returns the actual end of audio data. Updated all frame iteration functions to use this boundary instead of file size.

Changes:

• Added find_audio_end() in lib.rs
• Updated iterate_frames(), apply_gain(), apply_gain_channel(), apply_gain_wrap() to use audio_end boundary

Issue #55: M4A/AAC files incorrectly show MP3 format info

Problem: Running mp3rgain test.m4a (without -r or -a) showed incorrect MP3 format info.

Solution: Added M4A/AAC detection in process_info() to display appropriate format information.

Before:

test.m4a
  Format:      MPEG2.5 Layer III, Mono
  Frames:      1
  Gain range:  235 - 235 (avg: 235.0)

After:

test.m4a
  Format:      M4A/AAC
  Note: Use -r or -a for ReplayGain analysis

Testing

• All existing tests pass
• Manually tested with MP3 file containing APE tag - gain range correctly changes from 72-210 to 69-207 after applying -3 steps
• Manually tested M4A file info display - shows correct format info

Fixes #54
Fixes #55