Render the three per-note teaching marks on both highways, mirroring the
bend-curve render (#532). Display only — no scoring / NoteVerifier coupling.
- 2D static/highway.js: fg renders by default as a small finger numeral hugging
the gem (T = thumb, 1..4); sd (degree label) and ch (strum bracket connecting
notes that share a ch key, arrow direction from pkd) are opt-in behind a new
`showTeachingMarks` toggle (exposed via toggle/get/set + the bundle's
`teachingMarksVisible` flag). Pure helpers teachingFingerLabel /
teachingDegreeLabel / strumGroupBuckets drive the glyphs. ch bracket is
note-stream-only (chord notes already read as one gesture).
- 3D plugins/highway_3d/screen.js: fg (default) + sd (opt-in, mirrors the 2D
toggle via bundle.teachingMarksVisible) render next to the per-note fret label
via a new pooled sprite (pTeachMarkLbl); _scrChordNote resets fg/sd so chord
notes don't inherit stale marks. ch strum brackets are deferred in 3D (no
cross-note batch pass in the per-note render); 2D covers ch.
Tests: tests/js/highway_teaching_marks.test.js extracts the pure helpers from
both files (extract-and-eval) and asserts label mapping + strum-group bucketing.
Part of got-feedback/feedback#334
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
