Add WAV/MP3 input with automatic 48 kHz resampling

[Copilot]

The --src-audio (cover mode) and neural-codec --encode paths only accepted WAV at exactly 48 kHz. This adds transparent WAV + MP3 support at any sample rate, auto-resampled to the 48 kHz the VAE encoder requires — no ffmpeg pre-conversion needed.

New: src/audio.h

Single header providing read_audio(path, T_audio, n_channels):

• Format detected by extension: .mp3 → dr_mp3, anything else → dr_wav
• Channel layout preserved as-is from the source file — no up/down-mix
• Linear resampler (audio_resample_linear) is channel-agnostic; only runs when sr ≠ 48000
• Returns malloc'd interleaved float [T × n_ch]; caller frees

New: thirdparty/

• dr_wav.h v0.14.5 — WAV decode (public domain / MIT-0, mackron/dr_libs)
• dr_mp3.h v0.7.3 — MP3 decode via minimp3 (public domain / MIT-0)

Zero new link-time dependencies — both are single-header, included once per translation unit via #define DR_*_IMPLEMENTATION inside audio.h.

Tool changes

• neural-codec.cpp: encode path switches read_wav() → read_audio(); emits a warning if the decoded channel count ≠ 2
• dit-vae.cpp: --src-audio switches to read_audio(); same channel-count warning; help text updated
• CMakeLists.txt: thirdparty/ added as SYSTEM include in the shared link_ggml_backends macro (vendor warnings suppressed)

Example

# Before: required exact 48 kHz stereo WAV, manual ffmpeg conversion otherwise
# After: any of these just work
./dit-vae --src-audio reference.mp3   ...
./dit-vae --src-audio reference.wav   ...  # any sample rate
./neural-codec --vae vae.gguf --encode -i song.mp3 -o song.latent

New example

examples/cover.sh + examples/cover.json — demonstrates cover-mode generation from a WAV or MP3 reference track with inline usage notes.

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