mruby-dir: fix C4244 warning in mrb_dir_getwd function

use mrb_int for size variable and cast to size_t only when calling getcwd.
this maintains consistency with mruby type system while avoiding
conversion warnings on windows vc compiler.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-dir/src/dir.c

@@ -178,7 +178,7 @@ mrb_dir_getwd(mrb_state *mrb, mrb_value klass)
   mrb_int size = 64;
 
   path = mrb_str_buf_new(mrb, size);
-  while (getcwd(RSTRING_PTR(path), size) == NULL) {
+  while (getcwd(RSTRING_PTR(path), (size_t)size) == NULL) {
     int e = errno;
     if (e != ERANGE) {
       mrb_sys_fail(mrb, "getcwd(2)");