fix: set default executor to node's 32-thread pool

asyncio.to_thread and run_in_executor(None) use the default executor
(5-8 threads). At 50+ nodes, concurrent thrall/RPC/sidecar calls
exhaust it — urllib's internal DNS/connection calls queue behind them,
making cockpit unresponsive. Share the node's existing 32+ thread pool.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Knarr-Forseti

src/knarr/dht/node.py

@@ -1611,6 +1611,9 @@ def _check_credit_restored(self, peer_public_key: str, old_balance: float, new_b
     async def start(self):
         """Starts the server and background tasks."""
         self._main_loop = asyncio.get_running_loop()  # TEST-02: capture running loop (not deprecated get_event_loop)
+        # Share node's 32+ thread pool as default executor — prevents asyncio.to_thread
+        # and run_in_executor(None) from starving on the default 5-8 thread pool at scale.
+        self._main_loop.set_default_executor(self._handler_pool)
         self.server = await asyncio.start_server(
             self._handle_connection, self._bind_host, self.node_info.port
         )