Set close_timeout=0 on exec WebSocket (~38x exec speedup)

[jhgaylor]

Closes #24.

Summary

websockets.connect() in WSCommand.start is called without an explicit close_timeout, so it inherits the library default of 10s. After the server sends the EXIT byte it holds the WebSocket open for ~5s before initiating its own close handshake — the client's await ws.close() blocks waiting for that reply, adding a fixed ~5s tax to every exec. Full root-cause analysis and phase-by-phase timings in #24.

Fix

Mirror the Elixir SDK's behavior. sprites-ex sends a CLOSE frame via :gun.ws_send on EXIT and immediately calls :gun.close to tear down the TCP — it never waits for the server's reciprocal CLOSE. close_timeout=0 in websockets-lib produces the same shape: the CLOSE frame is sent, but ws.close() returns immediately rather than awaiting the server.

 self.ws = await websockets.connect(
     url,
     additional_headers=headers,
     ping_interval=WS_PING_INTERVAL,
     ping_timeout=WS_PONG_WAIT,
     max_size=10 * 1024 * 1024,
+    close_timeout=0,
 )

Impact

Side-by-side bench against a warm sprite running cat /tmp/x (n=3, p50):

close_timeout	per-exec latency	speedup
10s (library default)	~5,300ms	—
0.5s	~650ms	8×
0	~140ms	38×

This puts sprites-py on parity with sprites-ex on the exec hot path. Cross-SDK bench (n=3, p50):

phase	sprites-py	sprites-ex
exec_first	128ms	156ms
exec_warm_avg	121ms	188ms

Behavior change

The client no longer waits for the server's reciprocal CLOSE frame. The CLOSE frame is still sent — the server will see a clean close — we just don't block our own coroutine waiting on it. There is no resource leak: the underlying TCP socket is torn down by websockets-lib's normal cleanup path regardless.

[Copilot]

Pull request overview

This PR addresses a consistent ~5s latency penalty on the WebSocket exec path by explicitly setting close_timeout=0 on the client WebSocket connection, so the client doesn’t wait for the server’s delayed reciprocal close handshake after receiving the EXIT frame.

Changes:

• Set close_timeout=0 on websockets.connect() in WSCommand.start() for exec connections.
• Add an in-code comment documenting the rationale and linking to the observed behavior/regression (#24).

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