Add SN_USE_WIN32_THREADS flag to force Windows API threading layer
when building packaged SDK, even with LLVM-MinGW which normally uses
winpthreads. This ensures the runtime .o files don't have external
pthread symbol references that fail when users don't have winpthreads.

- runtime_arena.h, runtime_thread.h/c: Check SN_USE_WIN32_THREADS
- compat_pthread.h: Activate when SN_USE_WIN32_THREADS defined
- compat_pthread.h: Fix timespec_get guard for MinGW compatibility
- CMakeLists.txt: Add -DSN_USE_WIN32_THREADS for Windows runtime build

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>