Scheduler GRANT_LIMIT separates admission threshold from FIFO depth

In the switch scheduler, GRANT_LIMIT is a hard admission gate that defaults to FIFO_DEPTH and must be <= FIFO_DEPTH. Setting it below FIFO_DEPTH leaves headroom so that an overflow in the FIFO is a failsafe case, not the normal path. The testbench exercises GRANT_LIMIT < FIFO_DEPTH with threshold, drain/resume, reservation-accounting, and batched-fairness tests. Check the scheduler parameter before assuming grants can fill the FIFO to its full depth.