test(gsd): add overlay backdrop and notification lock safety tests

- Overlay layout: verify backdrop dims base lines, no dim without flag,
  overlay composites on top of dimmed background
- Notification store: verify markAllRead and clearNotifications do not
  delete a foreign process's lock file

packages/pi-tui/src/__tests__/overlay-layout.test.ts

@@ -0,0 +1,65 @@
+// pi-tui — Overlay Layout Tests (backdrop dimming)
+
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { compositeOverlays, type OverlayEntry } from "../overlay-layout.js";
+
+function makeEntry(
+	lines: string[],
+	options?: OverlayEntry["options"],
+): OverlayEntry {
+	return {
+		component: { render: () => lines },
+		options,
+		hidden: false,
+		focusOrder: 1,
+	};
+}
+
+describe("compositeOverlays — backdrop", () => {
+	it("dims base lines when backdrop is true", () => {
+		const base = ["hello world", "second line"];
+		const overlay = makeEntry(["OVERLAY"], {
+			width: 7,
+			anchor: "top-left",
+			backdrop: true,
+		});
+
+		const result = compositeOverlays(base, [overlay], 20, 20, 2);
+
+		// All base lines in viewport should contain dim escape (\x1b[2m)
+		// The overlay line itself is composited on top, but underlying lines get dimmed
+		const dimmedLine = result.find((l) => l.includes("second line"));
+		assert.ok(dimmedLine, "should have a line containing 'second line'");
+		assert.ok(dimmedLine.includes("\x1b[2m"), "base line should be dimmed");
+	});
+
+	it("does not dim when backdrop is false/absent", () => {
+		const base = ["hello world", "second line"];
+		const overlay = makeEntry(["OVERLAY"], {
+			width: 7,
+			anchor: "top-left",
+		});
+
+		const result = compositeOverlays(base, [overlay], 20, 20, 2);
+
+		// Lines not covered by overlay should remain undimmed
+		const secondLine = result.find((l) => l.includes("second line"));
+		assert.ok(secondLine, "should have a line containing 'second line'");
+		assert.ok(!secondLine.includes("\x1b[2m"), "base line should not be dimmed");
+	});
+
+	it("overlay content renders on top of dimmed background", () => {
+		const base = ["aaaaaaaaaa"];
+		const overlay = makeEntry(["XX"], {
+			width: 2,
+			anchor: "top-left",
+			backdrop: true,
+		});
+
+		const result = compositeOverlays(base, [overlay], 10, 10, 1);
+
+		// The first line should contain the overlay text
+		assert.ok(result[0].includes("XX"), "overlay text should be composited");
+	});
+});

src/resources/extensions/gsd/tests/notification-store.test.ts

@@ -2,7 +2,7 @@
 
 import { describe, test, beforeEach, afterEach } from "node:test";
 import assert from "node:assert/strict";
-import { mkdtempSync, mkdirSync, rmSync, readFileSync, existsSync } from "node:fs";
+import { mkdtempSync, mkdirSync, rmSync, readFileSync, existsSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 
@@ -246,4 +246,37 @@ describe("notification-store", () => {
     assert.equal(getUnreadCount(), 0);
     assert.equal(getLineCount(), 0);
   });
+
+  test("markAllRead does not delete a foreign lock file", () => {
+    initNotificationStore(tmp);
+    appendNotification("msg1", "info");
+
+    // Simulate another process holding the lock
+    const lockPath = join(tmp, ".gsd", "notifications.lock");
+    writeFileSync(lockPath, String(Date.now()), "utf-8");
+
+    // markAllRead should still work (best-effort) but not delete the foreign lock
+    markAllRead();
+
+    assert.ok(existsSync(lockPath), "foreign lock file should not be deleted");
+
+    // Clean up the lock so afterEach doesn't leave artifacts
+    rmSync(lockPath, { force: true });
+  });
+
+  test("clearNotifications does not delete a foreign lock file", () => {
+    initNotificationStore(tmp);
+    appendNotification("msg1", "info");
+
+    // Simulate another process holding the lock
+    const lockPath = join(tmp, ".gsd", "notifications.lock");
+    writeFileSync(lockPath, String(Date.now()), "utf-8");
+
+    // clearNotifications should still work but not delete the foreign lock
+    clearNotifications();
+
+    assert.ok(existsSync(lockPath), "foreign lock file should not be deleted");
+
+    rmSync(lockPath, { force: true });
+  });
 });