vm.c: allow Hash subclasses to override []; close #6675

The OP_GETIDX optimization now only applies to instances of the Hash
class itself. Subclasses fall back to method dispatch, allowing them
to override the [] method. This fixes compatibility with libraries
like mruby-hashie that rely on aliasing/overriding [] in subclasses.

Trade-off: Hash#[] cannot be overridden on the Hash class itself
(only on subclasses). This is a reasonable semantic for mruby since
subclassing is the proper pattern for customization.

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

src/vm.c

@@ -1892,6 +1892,8 @@ RETRY_TRY_BLOCK:
         }
         break;
       case MRB_TT_HASH:
+        /* optimize only for Hash class; subclasses may override [] */
+        if (mrb_obj_class(mrb, va) != mrb->hash_class) goto getidx_fallback;
         va = mrb_hash_get(mrb, va, vb);
         ci = mrb->c->ci;
         regs[a] = va;