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.

src/module.c

@@ -1,3 +1,4 @@
+#include "platform.h"
 #include "stk.h"
 #include <stdint.h>
 #include <stdlib.h>
@@ -5,12 +6,6 @@
 
 #define STK_MOD_FUNC_NAME_BUFFER 64
 
-#ifdef _WIN32
-#define STK_PATH_SEP '\\'
-#else
-#define STK_PATH_SEP '/'
-#endif
-
 typedef int (*stk_init_mod_func)(void);
 typedef void (*stk_shutdown_mod_func)(void);