Closes the two open Phase 2 items and adds the custom-instruments fol…

[TwDover]

…low-up.

feat: phase 2 sample re-sourcing + custom-instruments second pass

Sample re-sourcing (7 of 13 synth-only voices restored, all velocity-layered):

• 4 basses (Karoryfer + Smolken double bass, CC0), 2 electric pianos
  (Sullivan Wurlitzer EP200 + Pianet T, CC-BY 3.0), string ensemble
  (VSCO 2 CE, CC0), sourced via the sfzinstruments org's SPDX-tagged libs.
• Fix: vibraphone shipped an octave too low (VCSL names middle C as C3);
  remapped F3–E6 and re-rendered. New sampleManifests.test.ts guards every
  set's pitch span so an octave slip fails CI instead of ears.
• build_velocity_samples.py gains FLAC input, per-spec octave shift (each
  verified by measuring the fundamental), per-note gain, stale-output pruning.
• Repair test_playback_voices.py (red since the license pass deleted samples/bass/).
• Sample tree 3.4 MB -> 20 MB.

Custom instruments (second pass):

• kind now selects the mapping shape and filters per-part options (was dead metadata).
• Drum kits: new customDrumKit.ts shape (GM pitch -> single-zone, never pitch-shifted),
  filename-matched import, synth-kit fallback for unfilled pieces.
• Instruments panel shows a style's real lineup with a this-style/all-styles scope toggle.
• Per-slot kit audition (audition.ts) catches mis-mapped files where they're edited.

Docs: roadmap, LICENSE_AUDIT, DATA_LICENSES, custom-instruments-design updated.