Second half of the #281 clock-enable set. CumSum is the generic
run-length accumulator with O(1) Neumaier-compensated state — the
branching Neumaier variant, not Sma's plain Kahan, so it stays exact
when a large running total absorbs a tiny sample (the standalone
accumulator has no windowed-magnitude guarantee; module doc explains
why the helper is not reused). When is the clock-enable driver: forwards
value iff gate, else a quiet cycle (eval->None), so every stateful
reducer composes gated unmodified — SMA(When(x,g),N) is the gated SMA.
Rosters CumSum and When (both count pins 31->33) and ships two engine
E2E fixtures: When forwarding/quiet-downstream through the real
GraphBuilder->compile->run seam, and a quiet When leg suppressing a
whole Resample Barrier(0) group (the deliberate stall semantics).
refs #281
