refactor(pool): remove redundant Conn.closed atomic field

[cxljs]

The closed field was redundant with stateMachine's StateClosed state. Now IsClosed() and Close() only use the state machine for consistency.

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Note

Low Risk
Small refactor that consolidates close-state tracking to the existing state machine; minimal behavioral change but could affect edge cases if any code relied on the atomic flag being set independently of state transitions.

Overview
Removes the redundant Conn.closed atomic field from Conn and eliminates all reads/writes to it.

Conn.IsClosed() and Conn.Close() now rely exclusively on the state machine (StateClosed) to determine and mark closed status, ensuring a single source of truth for connection closure.

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[ndyakov]

👍

[ndyakov]

Looks good to me, thanks.