mruby-dir: unify int and mrb_int types for consistency

Change int variables to mrb_int in mrb_dir_getwd and mrb_dir_chroot
to maintain consistent use of mruby's integer type internally.
Keep explicit casts only at system interface boundaries where
different types are required by system calls.

Eliminates VC warning C4267 while following the same type
unification approach used in pack.c.

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

mrbgems/mruby-dir/src/dir.c

@@ -246,7 +246,7 @@ mrb_dir_chroot(mrb_state *mrb, mrb_value self)
   return mrb_fixnum_value(0);
 #else
   const char *path;
-  int res;
+  mrb_int res;
 
   mrb_get_args(mrb, "z", &path);
   res = chroot(path);