Move lego mode from --lego <track> CLI flag to "lego" JSON request field

[Copilot]

Lego mode was configured via a CLI-only --lego <track> flag, inconsistent with all other generation parameters which live in the request JSON. This PR moves the track name into the request JSON as "lego": "<track>", enabling batch processing, persistent configs, and parity with repainting_start/repainting_end.

Request struct (src/request.h, src/request.cpp)

• Added std::string lego field (default "") to AceRequest
• Parse/write/dump support added alongside existing fields
• "lego" is omitted from written JSON when empty (consistent with optional fields)

CLI (tools/dit-vae.cpp)

• Removed --lego <track> option, lego_track variable, and CLI-level validation
• Validation moved to per-request scope: requires --src-audio, base model, valid track name
• audio_cover_strength forced to 1.0 automatically when "lego" is set
• Empty caption now allowed when "lego" is non-empty

Examples

• examples/lego.json: added "lego": "guitar", "lyrics": "[Instrumental]", removed manual audio_cover_strength
• examples/lego.sh: removed --lego guitar CLI flag

README

• Updated lego section to reflect JSON-based usage
• Added "lego": "" to the JSON reference block
• Added field documentation with valid track names and recommended settings

Usage:

{
    "caption": "electric guitar riff, funk, house music, instrumental",
    "lyrics": "[Instrumental]",
    "lego": "guitar",
    "inference_steps": 50,
    "guidance_scale": 7.0,
    "shift": 1.0
}

./build/dit-vae \
    --src-audio backing-track.wav \
    --request 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

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