Emits the value its series input carried 'lag' fired cycles ago, held in a
node-owned ring (lookbacks()=[1], the past in node state not the input column).
Warm-up is skip-emit (None for the first 'lag' cycles — never a fabricated
sentinel). Output is a pure function of pre-cycle state, the precondition for a
future engine to close a feedback loop (the RTL register C5 names). prevHigh15 =
Delay[1] on high15. Build-step 4 of milestone 'Strategy node vocabulary I'.
closes#87
