Engine: LilyPond subset -> Score -> Type 1 SMF with running status,
validated by four invariants; 253 tests, exactly one known issue
(issue #1, Clair de Lune lhDown register drift, kept loud on purpose).
App: library, piece detail (build/play/tempo/export), diagnostics,
credits; page-object UI flows.
Course: PLAN.md syllabus, docs/lessons bug stories, HISTORY.md
narrative, README course map.