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docs(signals): lean posture landed — contract answers, tripwire cap 14 → 4 (#3654)

Closes out the lean-posture proposal in
documentation/plans/responsiveness-findings-plan.md §Cost (landed with
with the gate as shipped — `wantsRerun() = log || folds.length > 0 ||
OBSERVE.records.observed("rerun")`, read at recomputeStart, honoured at
recordRerun — and the four contract questions answered:

- history("rerun") is empty for runs made while nothing wanted a record
  and holds every record from the moment something did; why() is a view
  of that buffer and shares its gate; subscriptions() reads the graph and
  is unaffected. No enable({ reruns: true }) — document, no option.
- OBSERVE.records.subscribe("rerun", …) turns record-building on from the
  next run start and off on unsubscribe; log and a registered fold do the
  same. The bare attribution.subscribe went with #3644.
- The checks read the frame, not the record: checkEffectCycle,
  checkRelayTear, checkHotRuns, checkHotTime, checkWastedRecompute,
  checkDepWidth all run before the gate.
- @sentry/solid-2 dropping its rerun subscription is the follow-up in
  getsentry/sentry-javascript#24517.

JSDoc on why()/subscriptions() and the diagnostics doc lines say the
same; attribution-lean-gate.test.ts pins WASTED_RECOMPUTE without a
record and why()/subscriptions() on a lean engine.

Tripwire (attribution-engine-cost.test.ts) re-measured 2026-09-24 under
vitest, six runs: folded 1.98–2.14x, listened 1.91–2.11x, lean
1.71–1.90x. The section's 490/55 ns ≈ 9x table was a different
flush-per-write micro-harness; the tripwire's numbers are the baseline
from here. Cap 14 → 4 (~85–100% headroom; trips when the engine costs
roughly double what it does today).

Co-authored-by: Claude via Cursor <cursoragent@cursor.com>

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