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/stk.c

@@ -1,4 +1,5 @@
 #include "stk.h"
+#include "platform.h"
 #include "stk_log.h"
 #include <stdint.h>
 #include <stdio.h>
@@ -49,7 +50,7 @@ static void build_path(char *dest, size_t dest_size, const char *dir,
 {
 	dest[0] = '\0';
 	strncat(dest, dir, dest_size - 1);
-	strncat(dest, "/", dest_size - strlen(dest) - 1);
+	strncat(dest, STK_PATH_SEP_STR, dest_size - strlen(dest) - 1);
 	strncat(dest, file, dest_size - strlen(dest) - 1);
 }