gc.c: use actual work done for debt repayment in incremental step

Decrement gc_debt by the actual number of objects processed
instead of the fixed GC_STEP_SIZE. This makes step_ratio
directly affect debt repayment: larger steps repay more debt,
naturally reducing GC invocation frequency.

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

src/gc.c

@@ -1332,7 +1332,7 @@ incremental_gc_step(mrb_state *mrb, mrb_gc *gc)
       break;
   }
 
-  gc->gc_debt -= (mrb_int)GC_STEP_SIZE;
+  gc->gc_debt -= (mrb_int)result;
 }
 
 static void