M3 machinery for BLOCKER #138: sink reductions can now fold online instead of
buffering every per-cycle row to an unbounded channel.
- aura-core: additive `Node::finalize` end-of-stream hook (default no-op) +
`SeriesFold` online accumulator (last / max_drawdown / sign_flips).
- aura-engine: `Harness::run` calls `finalize` once per node in topo order after
the source loop; `summarize` refactored to drive `SeriesFold` (byte-identical),
the now-dead `sign0` removed.
- aura-std: `GatedRecorder` (emits only gated rows + the final row on finalize)
and `SeriesReducer` (folds one f64 series, emits one summary row on finalize),
both emitting the `Recorder` channel type — no aura-engine dependency.
- Integration tests: folded reductions are byte-for-byte equal to the
raw-recorder path; the R-reduction's peak memory is O(trades), independent of
cycle count.
Ratified two off-plan fixes the implementer made in the tests (both plan defects,
not code defects): the GatedRecorder correctly flushes the final non-gated row on
finalize, so the retained count is K_TRADES + 1; and the heterogeneous PM record
needs kind-correct zero scalars (a blanket f64 zero is rejected by the kind-typed
AnyColumn::push).
Wiring these reducers into the stage1-r sweep (the actual O(cycles)->O(trades)
win) follows next. refs #138
Brummel
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