Support WebSocket over Unix domain sockets

Parent: #66

Port Bun corpus position 4219 (js/web/websocket/websocket-unix.test.ts).

Scope:

• ws+unix:// and wss+unix:// transports
• request path/query parsing and localhost Host behavior
• text and binary messages / binaryType
• missing socket and malformed URL failures
• TLS and subprocess coverage
• structured malformed-URL diagnostics (ERR_WEBSOCKET_UNIX_URL, operation, phase, cause, and source-aware stack)

Acceptance: 7 passed, 0 todo, 0 failed, 0 unsupported, 0 allowed-empty.

Implementation: 229ec5e

Implemented on main in 229ec5e.

Verified the exact Bun corpus file: 7 passed, 0 todo, 0 failed, 0 unsupported, 0 allowed-empty. The malformed URL path also has a permanent structured-error regression covering code, operation, phase, cause, and source-aware stack.