tests: the moe_mixed import guard must locate the module, not the repo

The 0.8.10 release train failed in 'verify installed wheel': the new guard
test opened gridbook/moe_mixed.py at a path relative to the test file, which
exists in a checkout but not in the release job's layout (tests copied to a
temp cwd, gridbook imported from the installed wheel). Resolve the source via
importlib.util.find_spec("gridbook.moe_mixed").origin instead — identical
in-repo, correct against any installed layout. Verified both ways: 73 passed
in-repo; the guard passes from a foreign cwd with only the package on the
path. Nothing was published from the failed train (publish + GitHub release
were skipped), so the v0.8.10 tag is re-cut on this commit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TZbSLixmqwxwbSV6v1MApK

Author: RobTand

CHANGELOG.md

@@ -13,7 +13,9 @@
   runner; the file now skips only for a missing vLLM and fails loudly on
   gridbook's own import errors, and a CPU-suite guard statically parses
   `moe_mixed`'s imports from `moe_gemv_select` and holds them against the
-  real module.
+  real module (resolved through `importlib.util.find_spec`, so the guard also
+  runs in the release train's installed-wheel layout, where the repo tree is
+  not at a path relative to the test file).
 - The mixed method's FP8-v2 dispatch gate is tri-state-correct: `require`
   keeps the exact fail-load A/B-arm refusal on mixed FP8 groups, `auto` (the
   unset default) keeps the inherited kernel for them with the reason

tests/test_cb_gemv_select.py

@@ -440,12 +440,17 @@ def test_moe_mixed_imports_only_names_moe_gemv_select_defines():
     symbol. Parse the import statically and hold it against the real module,
     no vLLM needed."""
     import ast
+    import importlib.util
     from pathlib import Path
 
     import gridbook.moe_gemv_select as sel
 
-    src = Path(__file__).resolve().parents[1] / "gridbook" / "moe_mixed.py"
-    tree = ast.parse(src.read_text())
+    # Resolve the source through the import system, not the repo layout:
+    # the release train runs this suite against the installed wheel from a
+    # temp cwd where ../gridbook/moe_mixed.py does not exist.
+    spec = importlib.util.find_spec("gridbook.moe_mixed")
+    assert spec is not None and spec.origin, "gridbook.moe_mixed has no locatable source"
+    tree = ast.parse(Path(spec.origin).read_text())
     names = [
         alias.name
         for node in ast.walk(tree)