doctor: platform-aware engine.binary check — Windows has no X_OK bit (#178, fixes #141)

On Windows, os.access(path, os.X_OK) always returns True for any existing
file (NTFS has no execute bit; executability is governed by file extension,
not mode bits). So the test_non_executable_engine test — which chmods the
engine to 0o644 and expects 'fail' — could never pass on Windows.

Fixed doctor.py to use a platform-aware check: on Windows, any existing
file is executable; on POSIX, honor the mode bits via os.access(X_OK).

Fixed the test to assert the correct per-platform expectation: 'pass' on
Windows (chmod is a no-op for executability), 'fail' on POSIX.

Co-authored-by: woolcoxm <13604288+woolcoxm@users.noreply.github.com>

c/tests/test_doctor.py

@@ -104,7 +104,14 @@ class DoctorTest(unittest.TestCase):
         report = self.report(ram_gb=40)
         checks = self.checks_by_id(report)
 
-        self.assertEqual(checks["engine.binary"]["status"], "fail")
+        # On Windows chmod(0o644) does not remove executability (NTFS has no
+        # execute bit; os.access(X_OK) is always True for existing files), so
+        # the engine.binary check stays "pass" there. The excessive-RAM check
+        # (memory.ram) is platform-independent and must still fail. (#141)
+        if sys.platform == "win32":
+            self.assertEqual(checks["engine.binary"]["status"], "pass")
+        else:
+            self.assertEqual(checks["engine.binary"]["status"], "fail")
         self.assertEqual(checks["memory.ram"]["status"], "fail")
         self.assertEqual(report["status"], "error")