fix(relay): apply host Unix socket permissions

[stephenlclarke]

Summary

Apply UnixSocketConfiguration.permissions when an .outOf UnixSocketRelay creates its host-side listener.

The .into path already forwards the configured mode to UnixType, but .outOf omitted it and therefore used the process-default socket mode. A caller requesting a private endpoint such as 0600 could receive broader permissions than requested.

Extract the host-listener construction into a package-visible helper so the filesystem contract can be tested without a VM, pass the optional mode through, and close the new socket if listen() fails.

Testing

swift test --filter UnixSocketRelayTests

Result: 1 focused regression passed, verifying that an .outOf relay configured with 0600 creates a host socket with mode 0600.

Compatibility

Callers that omit permissions keep the existing default. Callers that provide a mode now receive the documented behavior consistently in both relay directions.