port(EterLib): Mutex/Thread/FileLoaderThread/GrpVertexBufferDynamic verbatim over Win32Crt kernel-object shims

Adds CreateSemaphore/ReleaseSemaphore/WaitForSingleObject/CloseHandle/
SetThreadPriority/_beginthreadex to Win32Crt; port.file_loader_thread covers
the shims, CThread and both the un-Created (40250 live) and Created loader.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/port/common/Win32Crt.h

@@ -73,6 +73,21 @@ BOOL FindNextFile(HANDLE handle, WIN32_FIND_DATA* data);
 BOOL FindClose(HANDLE handle);
 void Sleep(DWORD milliseconds);
 
+// kernel32 synchronization objects used by EterLib (FileLoaderThread): a counting semaphore and a
+// _beginthreadex thread, both waitable with WaitForSingleObject and freed with CloseHandle.
+// Only the unnamed, default-security forms exist; thread priority is accepted and ignored.
+#define WAIT_OBJECT_0 0x00000000L
+#define WAIT_TIMEOUT 0x00000102L
+#define WAIT_FAILED 0xFFFFFFFF
+#define THREAD_PRIORITY_NORMAL 0
+HANDLE CreateSemaphore(void* security_attributes, LONG initial_count, LONG maximum_count, LPCSTR name);
+BOOL ReleaseSemaphore(HANDLE semaphore, LONG release_count, LONG* previous_count);
+DWORD WaitForSingleObject(HANDLE handle, DWORD milliseconds);
+BOOL CloseHandle(HANDLE handle);
+BOOL SetThreadPriority(HANDLE thread, int priority);
+uintptr_t _beginthreadex(void* security, unsigned stack_size, unsigned (*start_address)(void*), void* arglist,
+	unsigned initflag, unsigned* thrdaddr);
+
 // MSVC <stdlib.h> __min / __max
 #ifndef __min
 #define __min(a, b) (((a) < (b)) ? (a) : (b))