feat(std): CumSum and When cells; roster complete at 33

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

crates/aura-cli/tests/graph_construct.rs

@@ -121,8 +121,8 @@ fn graph_introspect_vocabulary_lists_exactly_the_closed_roster_count() {
     let lines: Vec<&str> = stdout.lines().filter(|l| !l.is_empty()).collect();
     assert_eq!(
         lines.len(),
-        31,
-        "the std-only (no project) vocabulary has exactly the roster's 31 entries: {stdout}"
+        33,
+        "the std-only (no project) vocabulary has exactly the roster's 33 entries: {stdout}"
     );
 }