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_advanced_streaming.py

@@ -189,11 +189,16 @@ def test_dynamic_pub_sub(
     # sits forever until external SIGINT. The `afk_alarm_w_trace`
     # outer guard below is the AFK-safety counterpart (SIGALRM
     # raises in the main thread regardless of trio scope state).
-    fail_after_s: int = (
+    # scale the budget for slow/noisy CI runners (esp. macOS, where
+    # `cpu_scaling_factor()` applies a 3x CI bump) so a sluggish
+    # runner doesn't trip the deadline and inject a `TooSlowError`
+    # that collides with the `expect_cancel_exc=TooSlowError` param.
+    from .conftest import cpu_scaling_factor
+    fail_after_s: float = (
         8
         if is_forking_spawner
         else 20
-    )
+    ) * cpu_scaling_factor()
 
     async def main():
         # bug-class-3 breadcrumb: tag each level of the cancel path

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")