Add Constant Bit Rate (CBR) support for MP3Compressor

[sushmullur]

Implements CBR (Constant Bit Rate) support for MP3Compressor to complement the existing VBR (Variable Bit Rate) functionality, addressing issue #430.

Summary

Adds CBR mode support while maintaining full backward compatibility with existing VBR functionality. Users can now set constant bitrates from 32-320 kbps and
switch between VBR and CBR modes seamlessly.

API Changes

from pedalboard import MP3Compressor, MP3CompressorMode

# Existing VBR usage (unchanged)
vbr_compressor = MP3Compressor(vbr_quality=2.0)

# New CBR usage
cbr_compressor = MP3Compressor()
cbr_compressor.bitrate = 192  # Switches to CBR mode

# Mode switching
compressor = MP3Compressor(vbr_quality=3.0)  # VBR
compressor.bitrate = 128                      # Switch to CBR
compressor.vbr_quality = 1.0                  # Switch back to VBR

# Check current mode
print(compressor.mode)     # MP3CompressorMode.VBR or MP3CompressorMode.CBR
print(compressor.bitrate)  # Current CBR bitrate (when in CBR mode)

Implementation Details

C++ Changes (pedalboard/plugins/MP3Compressor.h):
- Added enum class Mode { VBR, CBR } for mode tracking
- Added setBitrate(), getBitrate(), getMode() methods with validation
- Updated prepare() to use appropriate LAME API calls:
  - VBR: lame_set_VBR(vbr_default) + lame_set_VBR_quality()
  - CBR: lame_set_VBR(vbr_off) + lame_set_brate()
- Added Python bindings for new properties and MP3CompressorMode enum

Type Hints (pedalboard_native/__init__.pyi):
- Added MP3CompressorMode enum class
- Added type hints for bitrate and mode properties
- Updated docstring to document CBR support

Test Coverage (tests/test_mp3_compressor.py):
- 9 new test functions covering all CBR functionality
- Tests all valid bitrates (32-320 kbps)
- Tests invalid bitrate rejection, mode switching, and edge cases

Supported CBR Bitrates

All MPEG-1 and MPEG-2 standard bitrates: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320 kbps

Invalid bitrates throw descriptive error messages listing valid options.

Validation

- Backward compatibility: All existing VBR tests pass unchanged
- Uses standard LAME API calls for CBR functionality
- Comprehensive error handling with clear validation messages
- Cross-platform compatibility with existing MP3 sample rate restrictions