gp5: extract per-track clef (clefMode + bass-tuning inference)

[kaizenman]

Bug

GP5File.readTrackRSE reads three consecutive int32s from the track's RSE block as ??? placeholders. The first of those is the track's clef mode (12 forces bass clef), documented in alphaTab's Gp3To5Importer.readTrack.

Without extracting it, every GP5 track is reported as treble clef regardless of:

• explicit clef mode flag (e.g. bass clef setting in Guitar Pro)
• bass-tuning inference (low-tuned tracks should default to bass clef)
• percussion track status (should use neutral clef)

The writer is asymmetric too: writeTrackRSE hard-codes the clefMode int32 to 0, so round-tripping a bass-clef track silently corrupts its clef.

Reference: alphaTab

alphaTab's Gp3To5Importer.ts:553-558 shows clef extraction in readTrack:

// `12` for all tunings which have bass clefs
const clefMode = IOHelper.readInt32LE(this.data);
if (clefMode === 12 || tuning[tuning.length - 1] < Gp3To5Importer._bassClefTuningThreshold) {
    this._clefsPerTrack.set(newTrack.index, Clef.F4);   // bass clef
} else {
    this._clefsPerTrack.set(newTrack.index, Clef.G2);   // treble clef
}

Gp3To5Importer.ts:618 handles percussion:

newBar.clef = Clef.Neutral;   // percussion track → neutral clef

Gp3To5Importer.ts:458 documents the bass-tuning threshold:

// Guitar Pro 3-6 changes to a bass clef if any string tuning is below B1

Fix

• Add TrackRSE.clefMode field
• Add MeasureClef.neutral enum value for percussion tracks
• readTrackRSE / writeTrackRSE: read/write clefMode (no longer ignoring)
• readMeasure: infer Measure.clef using alphaTab's rule:
  • explicit clefMode == 12 → bass
  • percussion track → neutral
  • low tuning (lowest string < MIDI B1 = 35) → bass
  • default → treble

Verification

Added 4 round-trip tests in tests/test_conversion.py:

• testGp5TrackClefRoundtripBass — explicit bass clef preserved
• testGp5PercussionTrackUsesNeutralClef — percussion → neutral
• testGp5LowTuningInfersBassClef — low tuning → bass
• testGp5GuitarTrackKeepsTrebleClef — guitar track stays treble

Without the fix: 3/4 tests fail (bass clef becomes treble; percussion not handled; low tuning not inferred).

With the fix: 4/4 tests pass.

All 196 existing tests continue to pass.

[Perlence]

Interesting, you can write 0 and 12 to show the grand staff.

[Perlence]

@kaizenman I went with a slightly different implementation, but thanks for the heads-up!