Two-level `(mode → input → action)` lookup. `set` validates each whitespace-
separated chord token via `InputKey::parse` and the action name via
`Action::parse`, so a bad TOML entry surfaces a precise diagnostic at TUI
startup instead of silently dropping a binding.
`known_bindings(mode)` feeds `ChordState` so a key only holds when it strictly
prefixes a real binding.
8 unit tests passing.
naps62-yolo
231032aa85