[samuel-williams-shopify]

Add directional socket shutdown, consuming socket close, and their admission-only _submit variants to Socket. Implement the same contract for native, Tokio, io_uring, and simulated backends, with a commented request/response half-close example.

Cleanup reuses the bounded pool and graceful draining already used for file close. Ordinary futures await completion and report execution errors; _submit futures await acceptance only. Shutdown borrows and privately retains the socket while accepted work runs. Close consumes exclusive ownership and rejects outstanding backend use rather than cancelling or joining operations. Applications must finish or cancel-and-await outstanding work before closing.

Socket close and shutdown currently use cleanup-pool syscalls even with io_uring selected; native SQEs remain a later optimization. Windows close preserves positive linger by switching the deregistered socket to blocking mode on the cleanup worker. Shared conformance and lifecycle tests cover half-close, pending reads, cancellation, admission, abandonment, and shutdown draining.