feat(tui): overlay positioning API with CSS-like values

Add OverlayOptions for configurable positioning (anchor, margins, offsets,
percentages). Add OverlayHandle for programmatic visibility control with
hide/setHidden/isHidden. Add visible callback for responsive overlays.

Extension API: ctx.ui.custom() now accepts overlayOptions and onHandle callback.

Examples: overlay-qa-tests.ts (10 test commands), doom-overlay (DOOM at 35 FPS).

packages/tui/src/index.ts

@@ -76,8 +76,10 @@ export {
 	type Component,
 	Container,
 	type OverlayAnchor,
+	type OverlayHandle,
 	type OverlayMargin,
 	type OverlayOptions,
+	type SizeValue,
 	TUI,
 } from "./tui.js";
 // Utilities