feat: add Pi conversation worker pool

tests/unit/pi-rpc-foundation.test.ts

@@ -233,12 +233,15 @@ describe('Pi worker process', () => {
 
   it('settles every pending command after an unexpected exit', async () => {
     const worker = await makeWorker();
+    const invalidations: string[] = [];
+    worker.subscribeInvalidation((error) => invalidations.push(error.code));
     const pending = worker.request({ type: 'no_response' });
     const crash = worker.request({ type: 'crash' });
 
     await expect(Promise.all([pending, crash])).rejects.toMatchObject({ code: 'PI_RPC_EXITED' });
     expect(worker.pendingCommandCount).toBe(0);
     expect(worker.generation).toBe(2);
+    expect(invalidations).toEqual(['PI_RPC_EXITED']);
   });
 
   it('keeps only bounded redacted stderr diagnostics', async () => {