Closed list of TUI verbs. The string form (`EnterInsert`, `FocusPane(Left)`,
`GotoTab(3)`) is what users write in `[tui.keymap.normal]`; the enum is what
the event loop matches on.
Parser surfaces `Unknown`, `BadArg`, and `Malformed` diagnostics so a bad
keymap entry is rejected at load time with a specific reason. Display round-
trips so the canonical form is stable.
9 unit tests passing.
naps62-yolo
a4252438cd