Add LEGO mode: generate instrument stems over a backing track

[Copilot]

Adds --lego <track> to dit-vae, enabling ACE-Step's LEGO pipeline: given a source audio file (--src-audio), generate a new instrument stem conditioned on it. Requires the base model (acestep-v15-base); turbo/SFT don't support it.

Changes

• tools/dit-vae.cpp — --lego <track> flag; constructs a "Generate the {track} track based on the audio context:" instruction injected into the DiT alongside --src-audio. Validates that --src-audio is present when --lego is used.
• tools/ace-qwen3.cpp — simplify phase-2 batch generation: replace dynamic compaction with a fixed N-batch forward (cond + uncond as 2*N), full CFG over all V_eff logits, mask the 24-token gap between EOS and audio codes with -1e9.
• examples/lego.json — preset (audio_cover_strength=1.0, inference_steps=50, guidance_scale=7.0).
• examples/lego.sh — self-contained 3-step pipeline: download base model → generate source track → apply lego.
• README.md — usage docs and available track name reference.
• .github/workflows/ci-build.yml — exclude ./mp3 and ./vendor from clang-format scan.

Usage

./build/dit-vae \
    --src-audio backing-track.wav \
    --lego guitar \
    --request examples/lego.json \
    --text-encoder models/Qwen3-Embedding-0.6B-Q8_0.gguf \
    --dit models/acestep-v15-base-Q8_0.gguf \
    --vae models/vae-BF16.gguf \
    --wav

Available tracks: vocals, backing_vocals, drums, bass, guitar, keyboard, percussion, strings, synth, fx, brass, woodwinds

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