fix(pi_rpc): detect a dead pi process immediately, not at the turn deadline

When pi died mid-turn, _read_stdout saw EOF and logged it, but nothing woke
_consume -- it sat on the event queue until asyncio.timeout(budget) fired at the
turn deadline (180 s). A dead process at second 3 was therefore reported as "no
reply" at second 180, and the fallback that launched a fresh pi hid the death
entirely. To the user this read as a hard hang.

_read_stdout now enqueues a "_process_exited" sentinel on EOF, and _consume
raises PiRpcError on it, so the failure path (same-session fallback model, then
rotation, then a fresh process) starts within milliseconds.

Smoke-tested: SIGKILL the child during a command and the client reports it in
0.0 s instead of the full budget, with no orphan left behind.

shared/lib/py/pi_rpc.py

@@ -445,7 +445,13 @@ class PiRpcClient:
                             await result
                     except Exception:
                         LOG.exception("on_event callback failed")
-        LOG.warning("pi RPC stdout closed")
+        # EOF: the child died or closed its pipe. Without a sentinel, _consume
+        # would sit on _events.get() until the turn deadline -- a process that
+        # exited at second 3 would be reported as "no reply" at second 180, and a
+        # fallback that launches a fresh pi would hide the death entirely. Put a
+        # sentinel so the waiter fails immediately instead.
+        LOG.warning("pi RPC stdout closed pid=%s rc=%s", self.pid, self._process.returncode)
+        await self._events.put({"type": "_process_exited"})
 
     async def _read_stderr(self) -> None:
         assert self._process and self._process.stderr
@@ -528,6 +534,13 @@ class PiRpcClient:
             event = await self._events.get()
             kind = event.get("type")
 
+            if kind == "_process_exited":
+                rc = self._process.returncode if self._process else None
+                raise PiRpcError(
+                    f"pi process exited (pid={self.pid}, returncode={rc}) while "
+                    f"awaiting {request_id}; stderr tail: {' | '.join(self._stderr_tail[-3:])}"
+                )
+
             if kind == "response" and event.get("id") == request_id:
                 response = event
                 if not wait_for_settled or not event.get("success"):