Pacing state for a remote target needs a per-target index, not a per-port scan

In tx5-style pacing, each (addr, sub) from a throttle/stop/resume response is matched to a target slot by a function that returns the first table entry whose address matches and whose sub-address matches or is marked any-sub. Any-sub entries are a bitmask, and an unmatched lookup returns the sentinel. This gives O(1) indexed per-target state at the cost of a destination set fixed at elaboration.