Scheduler grant gate: available = GRANT_LIMIT - occupancy - reserved

Per destination, the scheduler computes available[d] = GRANT_LIMIT - occupancy_in[d] - reserved_q[d]. When available hits zero, no grant_out bit fires for that destination and new requests receive THROTTLE instead. Round-robin selection starts from rr_start, which advances past the last granted source. Use these expressions to predict which sources get grants versus throttles.