fix: harden Pi lifecycle recovery

backend/test/maintenance-gate.test.ts

@@ -1,4 +1,7 @@
 import { test, expect } from "vitest";
+import { existsSync, mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import { MaintenanceBarrier } from "../src/runtime/maintenance-gate.js";
 
 test("activation waits for an in-flight admission lease and rejects later admissions", async () => {
@@ -17,3 +20,28 @@ test("activation waits for an in-flight admission lease and rejects later admiss
   gate.deactivate();
   expect(gate.acquire()).toBeTypeOf("function");
 });
+
+test("durable activation survives recreation and deactivation removes the marker first", async () => {
+  const directory = mkdtempSync(join(tmpdir(), "tht-maintenance-gate-"));
+  const marker = join(directory, "maintenance.json");
+  try {
+    const first = new MaintenanceBarrier(marker);
+    const release = first.acquire();
+    expect(release).toBeTypeOf("function");
+    const activation = first.activate();
+    expect(existsSync(marker)).toBe(true);
+    expect(first.acquire()).toBeUndefined();
+    expect(first.status()).toEqual({ active: true, admissions: 1 });
+    release?.();
+    await activation;
+    expect(first.status()).toEqual({ active: true, admissions: 0 });
+
+    const recreated = new MaintenanceBarrier(marker);
+    expect(recreated.status()).toEqual({ active: true, admissions: 0 });
+    recreated.deactivate();
+    expect(existsSync(marker)).toBe(false);
+    expect(recreated.status()).toEqual({ active: false, admissions: 0 });
+  } finally {
+    rmSync(directory, { recursive: true, force: true });
+  }
+});