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/utils.ts

@@ -112,6 +112,8 @@ export function visibleWidth(str: string): number {
 		clean = clean.replace(/\x1b\[[0-9;]*[mGKHJ]/g, "");
 		// Strip OSC 8 hyperlinks: \x1b]8;;URL\x07 and \x1b]8;;\x07
 		clean = clean.replace(/\x1b\]8;;[^\x07]*\x07/g, "");
+		// Strip APC sequences: \x1b_...\x07 or \x1b_...\x1b\\ (used for cursor marker)
+		clean = clean.replace(/\x1b_[^\x07\x1b]*(?:\x07|\x1b\\)/g, "");
 	}
 
 	// Calculate width
@@ -160,6 +162,18 @@ export function extractAnsiCode(str: string, pos: number): { code: string; lengt
 		return null;
 	}
 
+	// APC sequence: ESC _ ... BEL or ESC _ ... ST (ESC \)
+	// Used for cursor marker and application-specific commands
+	if (next === "_") {
+		let j = pos + 2;
+		while (j < str.length) {
+			if (str[j] === "\x07") return { code: str.substring(pos, j + 1), length: j + 1 - pos };
+			if (str[j] === "\x1b" && str[j + 1] === "\\") return { code: str.substring(pos, j + 2), length: j + 2 - pos };
+			j++;
+		}
+		return null;
+	}
+
 	return null;
 }