fix: guard POSIX headers and flock test in test_bmi_cache.cpp for Windows

sys/file.h and unistd.h don't exist on Windows. Guard with #if !_WIN32.
Also guard the PopulateSkipsWhenLockHeld test which uses flock().

Author: Sunrisepeak

tests/unit/test_bmi_cache.cpp

@@ -1,7 +1,9 @@
 #include <gtest/gtest.h>
 #include <fcntl.h>
+#if !defined(_WIN32)
 #include <sys/file.h>
 #include <unistd.h>
+#endif
 
 import std;
 import mcpp.bmi_cache;
@@ -200,8 +202,10 @@ TEST(BmiCache, PopulateFailsIfBuildOutputMissing) {
     EXPECT_NE(pop.error().find("expected build output missing"), std::string::npos);
 }
 
+#if !defined(_WIN32)
 // M4 #9: when an external holder takes the .lock, populate_from must skip
 // (returns success but does NOT clobber the directory).
+// Uses flock() which is POSIX-only.
 TEST(BmiCache, PopulateSkipsWhenLockHeld) {
     Tmp t;
     auto home    = t.path / "home";
@@ -233,3 +237,4 @@ TEST(BmiCache, PopulateSkipsWhenLockHeld) {
     ASSERT_TRUE(pop2) << pop2.error();
     EXPECT_TRUE(std::filesystem::exists(k.manifestFile()));
 }
+#endif // !defined(_WIN32)