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>

doc/internal/gc.md

@@ -131,7 +131,9 @@ limit = (GC_STEP_SIZE / 100) * step_ratio
 ```
 
 With default `step_ratio = 200` and `GC_STEP_SIZE = 1024`, the
-limit is 2048 objects per step.
+limit is 2048 objects per step. After each step, `gc_debt` is
+decremented by the actual number of objects processed, so larger
+steps repay more debt.
 
 When the gray stack is exhausted, the final marking phase re-marks
 the arena and global variables to catch objects created during

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