feat(core): teaching marks fg/ch/sd — wire + GP import + sd derivation (§6.2.2) (#536)

Add the three OPTIONAL per-note feedpak 1.5.0 teaching marks — fg (fret-hand
finger), ch (strum-group key), sd (scale degree) — to the Note model and wire
format, mirroring the bend-shape work (#531). These are DISPLAY/TEACHING ONLY:
nothing in the scoring / note-verification path reads them.

- lib/song.py: Note.fret_finger / strum_group / scale_degree, default-omitted
  on the wire (fg/ch/sd) and decoded via _wire_int_optional; _parse_note reads
  the GP-written fretFinger XML attr. Pure helpers key_to_tonic_pc (§7.7 key
  name -> tonic pitch class) + scale_degree_for_pitch, plus base_open_string_midis
  / pitch_from_base / note_pitch_midi (tuning offsets + capo + fret -> MIDI,
  mirroring app.js _TUNING_BASE_MIDI).
- lib/gp2rs.py: GP5 note.effect.leftHandFinger -> fg (RsNote field + fretFinger
  XML attr), reusing the chord Fingering value convention.
- lib/gp2rs_gpx.py: GP8/GPIF per-note <LeftFingering> (p-i-m-a-c letter codes,
  verified against real GP8 exports) -> fg.
- server.py highway_ws: derive sd for notes + chord notes from the active
  keys.json key + sounding pitch when the author didn't author one (author value
  wins); base hoisted out of the per-note loop.

Tests: round-trip + omit-when-default + malformed-tolerance for fg/ch/sd;
key_to_tonic_pc + scale_degree_for_pitch + note_pitch_midi (standard/drop-D/
capo/bass) units; GP5 leftHandFinger and GP8 <LeftFingering> import.

Part of got-feedback/feedback#334

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_gp2rs.py

@@ -1220,6 +1220,33 @@ def test_chord_diagram_fingers_extracted():
     assert [ct.get(f"finger{i}") for i in range(0, 4)] == ["-1"] * 4
 
 
+def test_single_note_left_hand_finger_imports_as_fg():
+    """A GP single note's leftHandFinger imports as the `fg` teaching mark and
+    survives convert_track XML → _parse_note → note_to_wire (§6.2.2)."""
+    from song import _parse_note, note_to_wire
+    note = _ct_note(guitarpro.NoteType.normal, gp_string=2, fret=5)
+    note.effect.leftHandFinger = guitarpro.Fingering.middle  # -> 2
+    beat = _ct_beat(tick=0, dur_value=4, notes=[note])
+
+    root = ET.fromstring(convert_track(_ct_song([beat]), track_index=0))  # noqa: S314
+    xn = root.findall(".//notes/note")[0]
+    assert xn.get("fretFinger") == "2"
+    assert note_to_wire(_parse_note(xn))["fg"] == 2
+
+
+def test_single_note_open_finger_omits_fg():
+    """Open/unset leftHandFinger leaves fg unset — no fabricated finger."""
+    from song import _parse_note, note_to_wire
+    note = _ct_note(guitarpro.NoteType.normal, gp_string=2, fret=5)
+    note.effect.leftHandFinger = guitarpro.Fingering.open  # -1 -> unset
+    beat = _ct_beat(tick=0, dur_value=4, notes=[note])
+
+    root = ET.fromstring(convert_track(_ct_song([beat]), track_index=0))  # noqa: S314
+    xn = root.findall(".//notes/note")[0]
+    assert xn.get("fretFinger") is None
+    assert "fg" not in note_to_wire(_parse_note(xn))
+
+
 def test_chord_without_diagram_has_blank_fingers():
     # A plain two-note chord (effect.chord is None) is unchanged: blank name,
     # all-(-1) fingers — no regression for diagram-less charts.