writeMidiFile: emit PART GUITAR / GHL instrument tracks

[elicwhite]

Adds fret-instrument emission to writeMidiFile.

• buildFretTrack: one MIDI track per instrument group (guitar / bass /
  guitarghl / bassghl / keys / guitarcoop / rhythm / GHL variants);
  multiple difficulties merge into the same track.
• emitFretNotes: 5-fret (95/83/71/59 diffStarts) and 6-fret/GHL
  (94/82/70/58) note-number mapping; open notes routed two ways:
  ENHANCED_OPENS mode (diffStart+0, preserves chord-with-open) when any
  group contains both open and non-open, else forceOpen-SysEx fallback
  (diffStart+1 + SysEx, collapses chords with open but safer next to
  animations at easy diffStart). Coalesces fret-note length with
  overlapping animations at the same tick:noteNumber via pre-computed
  seq slots so animation ordering survives re-parse.
• Force modifier ranges (forceHopo / forceStrum / forceTap) via
  emitFretModifierRanges — inverts resolveFretModifiers: natural HOPO
  state is re-derived with the MIDI threshold (1 + res/3), and a modifier
  emits only when the resolved flag disagrees. forceHopo = diffStart+6
  (5-fret) / +7 (GHL); forceStrum = +7 / +8; forceTap as SysEx.
• reconstructModifierRanges: collapses per-tick hit sets into minimal
  contiguous ranges over noteTicksInOrder.
• Instrument-wide emission: star power (MIDI 116), solo (MIDI 103),
  flex lanes (126/127 with per-difficulty LDS velocity), per-track text
  events, versus phrases (105/106), animations (with skip when the fret
  note above already emitted at the same tick:noteNumber), per-track
  unrecognizedMidiEvents.
• [ENHANCED_OPENS] text event when that mode is active.
• Helpers added: addSysExOnOff, reconstructModifierRanges,
  midiIsFretChord / midiIsSameFretNote / midiIsInFretNote.

Vocal tracks (PART VOCALS / HARM1-3) land in the follow-up PR.

Tests: 15 new test cases covering track naming, 5-fret + GHL
note-number mapping, open-note encoding (both modes), force modifier
emission (hopo/strum/tap + non-emission when natural matches),
instrument-wide sections, and round-trip through parseChartAndIni for
both 5-fret modifiers and GHL chord-with-open. 53 midi-writer tests,
397 total scan-chart tests passing at the tip.

[elicwhite]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add ChartDocument + writeChartFolder orchestrator #78 : 2 dependent PRs (#79 , #87 )
• Add same-format round-trip integration tests #77
• writeMidiFile: emit PART VOCALS / HARM1 / HARM2 / HARM3 tracks #76
• writeMidiFile: emit PART GUITAR / GHL instrument tracks #75 👈 (View in Graphite)
• writeMidiFile: emit PART DRUMS instrument tracks #74
• Add writeMidiFile() with TEMPO/EVENTS/unrecognized-track emission #72
• writeChartFile: emit instrument track sections #71
• Add writeChartFile() with Song/SyncTrack/Events/unrecognized-section emission #70
• Add writeIniFile() for song.ini emission #69
• Add createEmptyChart() counterpart to parseChartAndIni #68
• Add shared File type in interfaces.ts #86
• master

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