Keep `HAS_UDS` false on Windows

Modern Windows Python can expose `AF_UNIX`, so `trio.has_unix`
alone can register the UDS backend even though its credential and
lifecycle paths remain POSIX-only.

Gate `HAS_UDS` on `sys.platform` and make the Windows import smoke
step assert the TCP-only capability contract.

(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))

.github/workflows/ci.yml

@@ -160,7 +160,7 @@ jobs:
       # tractor` must succeed everywhere, and `HAS_UDS` reflects
       # platform capability (False on Windows, True on POSIX).
       - name: 'Smoke: import tractor'
-        run: uv run python -c "import tractor; from tractor.ipc._uds import HAS_UDS; print('import tractor OK | HAS_UDS=', HAS_UDS)"
+        run: uv run python -c "import sys; import tractor; from tractor.ipc._uds import HAS_UDS; assert sys.platform != 'win32' or not HAS_UDS; print('import tractor OK | HAS_UDS=', HAS_UDS)"
 
       - name: Run tests
         run: >

tractor/ipc/_uds.py

@@ -115,10 +115,12 @@ _SUN_PATH_LIMIT: int = (
 
 
 # single source of truth for "is the UDS backend usable on this
-# host?" — reuse `trio`'s `has_unix` (the same predicate that gates
-# `trio.open_unix_socket()`) rather than re-deriving an `AF_UNIX`
-# probe in every consumer module.
-HAS_UDS: bool = has_unix
+# host?" Windows can expose `AF_UNIX`, but this backend remains
+# POSIX-only until its credential and lifecycle paths are supported.
+HAS_UDS: bool = (
+    sys.platform != 'win32'
+    and has_unix
+)
 
 
 def unwrap_sockpath(