remove per-object gcnext pointer from MRB_OBJECT_HEADER, saving one
word (8 bytes on 64-bit) per object slot. the gray list for tri-color
marking is replaced by a fixed-size stack (MRB_GRAY_STACK_SIZE=1024)
in mrb_gc. when the stack overflows, a linear heap rescan recovers
gray objects.
object slot size: 48 -> 40 bytes (16.7% reduction on 64-bit).
benchmarks show up to 12% RSS reduction on object-heavy workloads
with neutral performance impact.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
31fea1709f