gc.c: replace threshold model with debt-based GC trigger

Replace the threshold-based GC trigger (gc->threshold vs gc->live)
with a debt model (gc->gc_debt). Each allocation increments debt;
each GC step decrements by GC_STEP_SIZE. When a cycle completes,
credit is proportional to live_after_mark * interval_ratio, giving
a natural feedback loop that adapts to allocation rate.

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

include/mruby/gc.h

@@ -45,7 +45,7 @@ typedef struct mrb_gc {
   mrb_bool gray_overflow:1;        /* gray stack overflowed; needs heap rescan */
   size_t live;                     /* count of live objects */
   size_t live_after_mark;          /* old generation objects */
-  size_t threshold;                /* threshold to start GC */
+  mrb_int gc_debt;                 /* <0:credit, >0:needs GC */
   size_t oldgen_threshold;         /* threshold to kick major GC */
   mrb_gc_state state;              /* current state of gc */
   int interval_ratio;