fix(pi): strengthen final release proof

tests/unit/pi-subagent.test.ts

@@ -86,6 +86,12 @@ describe('Pi subagent scheduler', () => {
     await expect.poll(() => running).toBe(4);
     expect(maxRunning).toBe(4);
     expect(processBudget.activeCount).toBe(4);
+    expect(scheduler.getDiagnostics()).toEqual({
+      activeChildPermits: 4,
+      waitingChildPermits: 4,
+      activeDispatches: 2,
+      activeParents: 2,
+    });
     gate.resolve();
     const [leftResult, rightResult] = await Promise.all([left, right]);
     expect(leftResult.details.tasks).toHaveLength(4);
@@ -93,6 +99,12 @@ describe('Pi subagent scheduler', () => {
     expect(opened).toHaveLength(8);
     expect(maxRunning).toBe(4);
     expect(processBudget.activeCount).toBe(0);
+    expect(scheduler.getDiagnostics()).toEqual({
+      activeChildPermits: 0,
+      waitingChildPermits: 0,
+      activeDispatches: 0,
+      activeParents: 0,
+    });
     await scheduler.close();
   });