[smoltafet]

Summary of Changes

• SocketCanDaemonBus gains a unix_socket_path argument: when given, it connects to socketcand through a Unix domain socket (AF_UNIX) instead of TCP, as requested in Socketcand UNIX Socket Support #2018. host/port are then neither required nor allowed.
• host and port are now optional (default None); passing neither address form, or a mix of both, raises CanInitializationError before any connection attempt. Platforms without AF_UNIX raise the same error.
• tcp_tune is disabled with a warning for Unix sockets (same pattern as the existing Windows case).
• connect_to_server now takes a single address (a (host, port) tuple or a socket path), which is what socket.connect() accepts for both families.
• Tests: a fake socketcand server in test/test_socketcand.py performs the real handshake plus a send/receive round-trip over both a Unix domain socket and TCP (regression), and covers the invalid configuration combinations.
• News fragment added under doc/changelog.d/.

Related Issues / Pull Requests

• Closes Socketcand UNIX Socket Support #2018

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• Other (please describe):

Checklist

• I have followed the contribution guide.
• I have added or updated tests as appropriate.
• I have added or updated documentation as appropriate.
• I have added a news fragment for towncrier.
• All checks and tests pass (tox).

Additional Notes

Verified locally: pytest test/test_socketcand.py passes (8 tests, including the 4 new ones), the public can.interface.Bus(interface="socketcand", channel="can0", unix_socket_path=...) route connects against the fake server, and black --check / ruff check (project pins) are clean on the changed files. Full tox was not run in my environment, and the interface .rst page was not extended — happy to add that if you prefer it in the same PR.