feat(tui): hardware cursor positioning for IME support

- Add Focusable interface for components that need hardware cursor positioning
- Add CURSOR_MARKER (APC escape sequence) for marking cursor position in render output
- Editor and Input components implement Focusable and emit marker when focused
- TUI extracts cursor position from rendered output and positions hardware cursor
- Track hardwareCursorRow separately from cursorRow for differential rendering
- visibleWidth() and extractAnsiCode() now handle APC sequences
- Update overlay-test.ts example to demonstrate Focusable usage
- Add documentation for Focusable interface in docs/tui.md

Closes #719, closes #525

packages/tui/src/index.ts

@@ -75,6 +75,9 @@ export {
 export {
 	type Component,
 	Container,
+	CURSOR_MARKER,
+	type Focusable,
+	isFocusable,
 	type OverlayAnchor,
 	type OverlayHandle,
 	type OverlayMargin,