Skip `test_ringbuf` at collection off-linux

`tests/test_ringbuf.py` imports `tractor.ipc._ringbuf` at module
top, which pulls in `tractor.ipc._linux` whose module-level
`ffi.dlopen(None)` raises `OSError` on Windows (and any non-linux
host). That fires at COLLECTION, before the module's existing
`pytestmark = pytest.mark.skip` can apply, so it aborts the whole
pytest session — the new `windows-latest` CI leg never gets past
collection.

- guard the module with `pytest.skip(allow_module_level=True)`
  gated on `platform.system() != 'Linux'`, placed before the
  crashing import — same idiom as `tests/devx/test_debugger.py`.
- the `eventfd`-based ringbuf backend is linux-only by design, so
  macOS skips cleanly too (previously it only skipped incidentally
  via the absent `cffi` optional dep).

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/test_ringbuf.py

@@ -1,10 +1,22 @@
 import time
+import platform
 
 import trio
 import pytest
 
 import tractor
 
+# `tractor.ipc._ringbuf` is built on linux `eventfd(2)`; importing
+# it pulls in `tractor.ipc._linux` whose module-level
+# `ffi.dlopen(None)` raises on non-linux. Skip the whole module at
+# COLLECTION before that crashing import runs (a `pytestmark` skip
+# is too late — markers apply only after the import succeeds).
+if platform.system() != 'Linux':
+    pytest.skip(
+        'ringbuf (eventfd) IPC is linux-only',
+        allow_module_level=True,
+    )
+
 # XXX `cffi` dun build on py3.14 yet..
 pytest.importorskip("cffi")