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/coding-agent/examples/extensions/doom-overlay/wad-finder.ts

@@ -0,0 +1,51 @@
+import { existsSync, writeFileSync } from "node:fs";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+// Get the bundled WAD path (relative to this module)
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const BUNDLED_WAD = join(__dirname, "doom1.wad");
+const WAD_URL = "https://distro.ibiblio.org/slitaz/sources/packages/d/doom1.wad";
+
+const DEFAULT_WAD_PATHS = ["./doom1.wad", "./DOOM1.WAD", "~/doom1.wad", "~/.doom/doom1.wad"];
+
+export function findWadFile(customPath?: string): string | null {
+	if (customPath) {
+		const resolved = resolve(customPath.replace(/^~/, process.env.HOME || ""));
+		if (existsSync(resolved)) return resolved;
+		return null;
+	}
+
+	// Check bundled WAD first
+	if (existsSync(BUNDLED_WAD)) {
+		return BUNDLED_WAD;
+	}
+
+	// Fall back to default paths
+	for (const p of DEFAULT_WAD_PATHS) {
+		const resolved = resolve(p.replace(/^~/, process.env.HOME || ""));
+		if (existsSync(resolved)) return resolved;
+	}
+
+	return null;
+}
+
+/** Download the shareware WAD if not present. Returns path or null on failure. */
+export async function ensureWadFile(): Promise<string | null> {
+	// Check if already exists
+	const existing = findWadFile();
+	if (existing) return existing;
+
+	// Download to bundled location
+	try {
+		const response = await fetch(WAD_URL);
+		if (!response.ok) {
+			throw new Error(`HTTP ${response.status}`);
+		}
+		const buffer = await response.arrayBuffer();
+		writeFileSync(BUNDLED_WAD, Buffer.from(buffer));
+		return BUNDLED_WAD;
+	} catch {
+		return null;
+	}
+}