Intercept keyboard events via xterm's custom key handler and emit
readline/zle escape sequences so bindings work regardless of terminfo:
- Alt/Option + Left/Right: word-wise cursor move
- Alt/Option + Backspace: delete previous word
- macOS Cmd + Left/Right: jump to line start/end
- macOS Cmd + Backspace: clear to line start
Uses `e.code` to stay correct on dead-key layouts, skips IME
composition, and excludes AltGr (ctrl+alt) on Windows/Linux.
