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_gpx.py

@@ -32,6 +32,7 @@ from gp2rs_gpx import (
     _inject_tones,
     _resolve_pending_slides,
     _gpx_bend_shape,
+    _gpif_left_fingering,
 )
 from gp2rs import RsNote
 
@@ -731,6 +732,32 @@ def test_note_vibrato_ignores_whammy_trembar_property():
     assert _note_has_vibrato(n, tp) is False
 
 
+# ── _gpif_left_fingering (GP7/GP8 per-note fret-hand finger -> fg) ───────────
+# GPIF stores a single note's fret-hand finger as a direct <LeftFingering>
+# child of <Note> (NOT a <Property>), with classical p-i-m-a-c letter codes —
+# verified against real GP8 exports (Open / I / M observed). Maps to the same
+# RS finger integers as the chord-diagram path (§6.2.2). Teaching mark only.
+
+@pytest.mark.parametrize("code, expected", [
+    ("Open", -1), ("P", 0), ("I", 1), ("M", 2), ("A", 3), ("C", 4),
+    ("i", 1), ("m", 2),                # case-insensitive
+    ("index", 1), ("ring", 3),         # word forms also accepted
+])
+def test_gpif_left_fingering_letter_codes(code, expected):
+    n = ET.fromstring(f'<Note id="1"><LeftFingering>{code}</LeftFingering>'
+                      '<Properties></Properties></Note>')
+    assert _gpif_left_fingering(n) == expected
+
+
+def test_gpif_left_fingering_absent_or_unknown_is_unset():
+    # No <LeftFingering> child, or an unrecognised value -> -1 (never fabricate).
+    assert _gpif_left_fingering(ET.fromstring('<Note id="1"/>')) == -1
+    assert _gpif_left_fingering(
+        ET.fromstring('<Note id="1"><LeftFingering>Z</LeftFingering></Note>')) == -1
+    assert _gpif_left_fingering(
+        ET.fromstring('<Note id="1"><LeftFingering></LeftFingering></Note>')) == -1
+
+
 # ── convert_file: GP8 chord-diagram name + fingering extraction (E3) ─────────
 # GP7/GP8 GPIF carries authored chord diagrams under a track's
 # Property[@name="DiagramCollection"]. Each Item gives the chord name and a