Vim-inspired modal input handling: translates key events into calls against the pure navigation functions in Termtab.UI.Types and the document edits in Termtab.UI.Editing, plus the stateful bits neither of those modules owns — playback transport, file save, and note preview.

Dispatch is keyed on asInputMode (NormalMode, GoToMode, VisualMode, a nested MenuMode, or CommandMode); each mode has its own handle* function below that only recognizes the keys valid in that mode, falling through to a "you're on your own" case in NormalMode or an explicit "unknown" status message in the others.