Review 02 P1 / Wave 2 rule (d): AbLegacy runs PollGroupEngine but raised only
the per-tag OnDataChange, so every changed tag paid its own trip through the
actor chain. The engine's per-cycle batch is now raised as OnDataChangeBatch
(one per poll cycle), alongside the per-tag event for legacy consumers.
Refs #738
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty