Keep overflow failsafe when adding a grant headroom gate

When a scheduler admits grants below the FIFO's physical depth, the gap (FIFO_DEPTH - GRANT_LIMIT) acts as headroom for writes that were granted against a stale occupancy reading. Do not remove the queue_overflow check just because grants are clamped earlier; it still catches protocol violations (writes without a grant) and scheduler lag. Keep the failsafe and treat its firing as an alarm rather than a normal event.