Feature 108 (US5, FR-016): the modifier state recovered at the key boundary. The raw key string can carry `Ctrl+`/`Alt+`/`Shift+`/`Meta+` prefixes that the plain `normalize` collapses into `Unknown "Ctrl+L"` and loses; parsing them here makes chords as dependable as plain keys, with no backend change. A closed record of four bools — total, deterministic, equatable.