sound polish, license compliance, custom instruments

[TwDover]

phase 2 — sound polish, license compliance, custom instruments

Sound polish:

• Master soft-clip limiter (Linux-safe WaveShaper, not a DynamicsCompressor)
  on all master paths: live, offline WAV render, MediaRecorder tap
• Per-style FX presets + cross-style loudness normalization
• Velocity-layer/round-robin engine (LayeredSampler + velocity.json manifest);
  vibraphone migrated to VCSL CC0 soft/hard layers via build_velocity_samples.py

License compliance:

• Full audit (docs/LICENSE_AUDIT.md); frontend + backend deps all GPL-compatible
• Removed MusyngKite bass/melodic samples (ambiguous license) and the unused,
  unlicensed Tone.js drum samples; dropped dead getDrumKit code + download_samples.py
• Removed voices synthesize; added a persisted Samples/Synth toggle
• Fixed package.json license GPL-3.0-only -> GPL-3.0-or-later to match headers

Custom instruments (MVP — needs desktop runtime test):

• Upload samples, auto-mapped to notes/velocities, assignable to any part
• Pure mapping core + library store + Electron IPC storage (blob: playback,
  no custom scheme) + Instruments panel UI and transport button
• See docs/custom-instruments-design.md

Frontend suite 12 -> 44 green; vue-tsc + eslint + production build clean.