engine: auto-derive chord-quality / structural / rhythm tags; parse pull-off, tap, let-ring

[PhysShell]

The tag taxonomy SwancoreTag (core/src/corpus.rs:79) already defines maj7/min7/sus2/add9/slash_chord/power_chord, intro/verse/chorus/bridge/interlude/outro, and syncopated — but there is no auto-derivation; these are manual/curator-only today. (Technique tags, by contrast, are auto-derived: core/src/technique.rs.)

Asks

• Chord quality (maj7/min7/power_chord, …): add harmonic/voicing analysis to set these from pitch-class / interval content per phrase. (User: "maj7/min7 как выставлять?" — currently only by hand.)
• Structure (intro/verse/chorus/bridge/interlude/outro): infer from repetition + section heuristics (available data: master_bars, density/register changes). Pairs with repeat detection — see the segmentation issue.
• syncopated: derive from onset-vs-grid placement.
• Missing technique parsing in core/src/gp.rs map_gp_note_marks (568–618): pull-off (the field exists in the guitarpro crate, just isn't mapped — easy), tap, and let-ring (no model variant yet — would need a new SpanTechnique).

Already parsed today: slide, bend, hammer_on, vibrato, palm_mute, accent, ghost, staccato, dead_note, natural/pinch harmonic.

Origin: playtest session 2026-06-19.