BBP2MIDI V15 – piano extended-note investigation build

This build is an investigation candidate, not yet a final release.

Change from V14:
- For PlayType.Piano events, first use a populated pianoOrig[N-1] voicing.
- If the event value is >= 40 and the corresponding pianoOrig slot is not populated,
  emit the value itself as a literal MIDI pitch.
- Existing populated pianoOrig slots remain unchanged.

Rationale:
- The BBP corpus contains genuine PlayType.Piano event values from 40 through 107.
- pianoOrig is only 64 entries long, and some songs contain unused/FF-filled pianoOrig
  slots in the 40-64 region while also using those event values.
- The 65-107 events cannot be pianoOrig indices at all.
- This makes a literal-pitch fallback a defensible hypothesis, but hardware validation
  is still required before calling it final.
