Live run against ab_server (slc500/micrologix/plc5 x libplctag/native): with
coalescing on, the blackholed device's 20 tags are one block, i.e. one comms
fault per poll, so its backoff window opens on the second poll rather than the
first. The gate now asserts the actual contract: at most two timeouts across
three polls, the live device Good on every poll, and the third poll fast.
Refs #738
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
34e455f412