Add file descriptor passing and fix socket teardown races

[colemancda]

Adds Unix file descriptor passing (SCM_RIGHTS) and fixes two socket teardown races found while building a pure-Swift D-Bus client on top of this package.

File descriptor passing

CMSG_FIRSTHDR, CMSG_NXTHDR, CMSG_DATA, CMSG_LEN and CMSG_SPACE are macros, so they are unavailable from Swift on every platform. CSocket gains two shims — c_socket_send_descriptors and c_socket_receive_descriptors — wrapping sendmsg/recvmsg with an ancillary buffer. These are the only C shims compiled on Darwin as well as Linux and Android.

On top of them:

• SocketMessage — received payload, accompanying descriptors, and whether the ancillary buffer was truncated.
• SocketDescriptor.send(_:fileDescriptors:) and SocketDescriptor.receive(_:maximumDescriptors:) — blocking, Result-returning, matching the existing descriptor API.
• Socket.write(_:fileDescriptors:) and Socket.receiveMessage(_:maximumDescriptors:) — async wrappers going through AsyncSocketManager, so a descriptor-carrying send waits for writability like any other write.

Two details worth calling out:

• Descriptors beyond maximumDescriptors are closed rather than leaked. The kernel has already installed them in this process by the time recvmsg returns, so dropping the buffer would leak them.
• Sending with an empty payload returns EINVAL. A zero-length message can be discarded before its ancillary data is delivered, so the descriptors would vanish silently.

Teardown fixes

AsyncSocketManager keys its state by file descriptor number, and those numbers are reused as soon as they are closed. Two consequences:

1. Deferred poll results applied to a reused descriptor. A poll result computed for the old socket could be applied to the new one occupying the same number, tearing down a healthy connection. Poll results are now checked against the current SocketState identity before they take effect, and a descriptor missing from the table is skipped instead of trapping.

2. A never-connected socket polls as POLLHUP. An AF_UNIX stream socket that has neither connected nor listened reports hangup — verifiable outside Swift entirely. Because Socket.init registers with the manager before connect, the monitor could hang up a socket that was about to be used. SocketState now tracks whether the socket was ever established, and hangup is only acted on for established or listening sockets.

Both were reproducible as intermittent ESHUTDOWN on a freshly opened socket under a concurrent test suite.

Tests

Eight tests covering round trips of one and several descriptors, descriptors surviving with payload data, truncation reporting, over-capacity descriptors being closed, and the empty-payload rejection.

Notes

• CI now also builds Linux ARM64, and the Swift floor is raised to 6.2.
• The pre-existing test failures on this repo (the force unwrap at NetworkInterface.swift:58, and the fixed-port and fixed-path tests colliding with leftovers from earlier runs) are untouched by this branch and fail the same way on main.