Add DSD (Direct Stream Digital) audio format support

[nixternal]

Add support for DSD audio formats (DSF, DFF) to enable DR analysis of high-resolution DSD files.

Implementation notes:

• DSD files need conversion to PCM before DR analysis
• Most common format is DSF
• Possible libraries: pydsf (simplest), or sox wrapper
• Should be optional dependency like pydub
• Need to decide on conversion sample rate (88.2kHz, 96kHz, 176.4kHz)

Acceptance criteria:

• Can analyze .dsf files
• Can analyze .dff files (stretch goal)
• DSD support is optional dependency
• Documentation updated
• Tests added