fix: resolve Windows segfaults by using platform-specific path separators

On Windows, module IDs were failing to load because paths were being
constructed with hardcoded forward slashes. Oops...

- Added platform.h to centralize path separator macros.
- Updated build_path in stk.c to use STK_PATH_SEP_STR instead of "/".
- Cleaned up redundant platform logic in module.c.

include/platform.h

@@ -0,0 +1,12 @@
+#ifndef STK_PLATFORM_H
+#define STK_PLATFORM_H
+
+#ifdef _WIN32
+#define STK_PATH_SEP '\\'
+#define STK_PATH_SEP_STR "\\"
+#else
+#define STK_PATH_SEP '/'
+#define STK_PATH_SEP_STR "/"
+#endif
+
+#endif /* STK_PLATFORM_H */