fix(tests): skip efficiency tests when glm_tiny fixture is absent (CI #360)

CI runs 'make check' = dependency-free tests, no model downloads (by design,
#140). glm_tiny/ is a gitignored generated fixture, so test_inefficiency.py
hard-failed on the Windows/macOS/Linux runners with 'config.json: No such file
or directory' instead of skipping.

_engine_present() now requires BOTH glm.exe AND glm_tiny/config.json, and
_skip_reason() names exactly which prerequisite is missing so the skip is
actionable. Verified: 8 skipped (0 failed) with the fixture absent; 5 pass +
3 CUDA-skip with it present.

c/tests/test_inefficiency.py

@@ -31,7 +31,25 @@ TINY = C_DIR / "glm_tiny"
 
 
 def _engine_present() -> bool:
-    return ENGINE.exists()
+    """True iff BOTH the built engine AND the tiny fixture are available.
+
+    These tests need glm.exe (a build artifact) AND glm_tiny/ (a generated
+    fixture, gitignored). CI runs `make check` = "dependency-free tests, no
+    model downloads" (workflow .github/workflows/check.yml, by design #140), so
+    neither is present there and these tests must SKIP rather than fail. They
+    run locally after `make glm.exe` (glm_tiny ships alongside the source, or
+    is regenerated by tools/make_glm_oracle.py).
+    """
+    return ENGINE.exists() and (TINY / "config.json").exists()
+
+
+def _skip_reason() -> str:
+    """Name exactly which prerequisite is missing, so the skip is actionable."""
+    if not ENGINE.exists():
+        return "glm.exe not built (run: make glm.exe)"
+    if not (TINY / "config.json").exists():
+        return "glm_tiny fixture absent (gitignored; ship it locally or run tools/make_glm_oracle.py)"
+    return ""
 
 
 def _cuda_available() -> bool:
@@ -66,7 +84,7 @@ def _cuda_available() -> bool:
         return False
 
 
-@unittest.skipUnless(_engine_present(), "glm.exe not built (run: make glm.exe)")
+@unittest.skipUnless(_engine_present(), _skip_reason() or "glm.exe + glm_tiny required")
 class TinyEfficiencyTest(unittest.TestCase):
     """Asserted regression tests on the resident tiny model. Gates CI."""
 
@@ -156,7 +174,7 @@ class TinyEfficiencyTest(unittest.TestCase):
 
 
 @unittest.skipUnless(_cuda_available(),
-                     "CUDA build not present (run: make clean && make glm.exe CUDA_DLL=1 && make cuda-dll)")
+                     _skip_reason() or "CUDA build not present (run: make clean && make glm.exe CUDA_DLL=1 && make cuda-dll)")
 class TinyCudaEfficiencyTest(unittest.TestCase):
     """CUDA-path regression tests on the tiny model. Skip unless CUDA built.