KOS_OP_FAST_NEXT

• Style: EXTRA (FASTVAR, COUNT). Row = iterator slot, n (1 or 2).

• Stack: As VAR_NEXT, with the iterator loaded from its fast slot.

• Compiler form: Post-compile only: the generic convert_fast lowering of VAR_NEXT when the iterator name has a fast slot.

• Implementation: koskari/lib/opcode/FAST_NEXT.c (cold handler)