Four distinct bugs in the acestep-cpp UI/backend integration: missing slider in reference mode, wrong LM default, ace-understand never installed to bin/, and understand results not persisted.

UI fixes (components/CreatePanel.tsx)

• lm_negative_prompt default: 'NO USER INPUT' → ''
• audio_cover_strength slider: now shown in the AUDIO: REFERENCE tab when a reference audio is loaded (was cover-only)
• Understand (✨) button + result panel: added to the Lego tab backing-track player; Reference and Cover tabs already had it
• handleUnderstand: routes to /:id/understand (persists to DB) when the audio URL matches an uploaded reference track, falls back to /understand-url otherwise

ace-understand binary not found (build.sh, build.bat)

ace-understand was never copied to ./bin/ post-build — only ace-qwen3, dit-vae, and neural-codec were. Config auto-discovery already looked in ./bin/; the copy step was just missing.

# before
for name in ace-qwen3 dit-vae neural-codec; do
# after
for name in ace-qwen3 dit-vae ace-understand neural-codec; do

Also added ACE_UNDERSTAND_BIN to .env.example override docs.

Metadata persistence (server/)

• Migration (migrate.ts): adds understand_metadata TEXT column to reference_tracks via a safe incremental ALTER TABLE (idempotent, catches already-exists).
• Route (referenceTrack.ts): POST /:id/understand now saves the result JSON to understand_metadata after analysis — best-effort, non-fatal on save failure.

Reference audio in spawn mode (server/src/services/acestep.ts)

When referenceAudioUrl is set in text-to-music mode, reference_audio and audio_cover_strength are now written into the request JSON passed to the binary — mirrors what the HTTP API path already did.

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