feat(tui): add OverlayOptions API and fix width overflow crash

Adds positioning/sizing options for overlays and fixes crash when compositing
lines with complex ANSI sequences exceed terminal width.

packages/tui/test/overlay-short-content.test.ts

@@ -0,0 +1,57 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import { type Component, TUI } from "../src/tui.js";
+import { VirtualTerminal } from "./virtual-terminal.js";
+
+class SimpleContent implements Component {
+	constructor(private lines: string[]) {}
+	render(): string[] {
+		return this.lines;
+	}
+	invalidate() {}
+}
+
+class SimpleOverlay implements Component {
+	render(): string[] {
+		return ["OVERLAY_TOP", "OVERLAY_MID", "OVERLAY_BOT"];
+	}
+	invalidate() {}
+}
+
+describe("TUI overlay with short content", () => {
+	it("should render overlay when content is shorter than terminal height", async () => {
+		// Terminal has 24 rows, but content only has 3 lines
+		const terminal = new VirtualTerminal(80, 24);
+		const tui = new TUI(terminal);
+
+		// Only 3 lines of content
+		tui.addChild(new SimpleContent(["Line 1", "Line 2", "Line 3"]));
+
+		// Show overlay centered - should be around row 10 in a 24-row terminal
+		const overlay = new SimpleOverlay();
+		tui.showOverlay(overlay);
+
+		// Trigger render
+		tui.start();
+		await new Promise((r) => process.nextTick(r));
+		await terminal.flush();
+
+		const viewport = terminal.getViewport();
+		const hasOverlay = viewport.some((line) => line.includes("OVERLAY"));
+
+		console.log("Terminal rows:", terminal.rows);
+		console.log("Content lines: 3");
+		console.log("Overlay visible:", hasOverlay);
+
+		if (!hasOverlay) {
+			console.log("\nViewport contents:");
+			for (let i = 0; i < viewport.length; i++) {
+				console.log(`  [${i}]: "${viewport[i]}"`);
+			}
+		}
+
+		assert.ok(hasOverlay, "Overlay should be visible when content is shorter than terminal");
+
+		tui.stop();
+	});
+});