experiment: compare acknowledge priority alternatives

Experiment 41 compared two passive alternatives for reducing Acknowledge dominance:

Variant A (priority reordering): evaluate Summarise/Pause before Acknowledge
- Converges on concluding→summarise and stalled→pause correctly
- Introduces false-positive summarise at long-investigation t3

Variant B (Acknowledge exclusions): gate Acknowledge via phase/progress/health
- Converges on the same two genuine changes without false-positives
- Recommended: cleaner boundaries, preserves Acknowledge for healthy focus states

Both variants produce identical results for 2 of 7 tested turns.
Variant A diverges at long-investigation t3 (focusing phase with resolvedNodeCount=3).
Variant B correctly preserves Acknowledge there via its exclusion list.

Test files:
- tests/behaviour-selection.counterfactual.test.js (44 tests, new)
No production code changed.

docs/current-project-state.md

@@ -46,7 +46,7 @@ These are what currently affect the working engine:
 The following were built during Experiments 18–25B. They are isolated diagnostic layers with no active integration into the user-facing investigation:
 
 - Investigation-state assessment (phase and progress classification);
-- Behaviour selection from assessed state — passively evaluated in Experiment 39 against real assessment outputs; produces all valid behaviours but Acknowledge dominates when health=healthy;
+- Behaviour selection from assessed state — passively evaluated in Experiments 39–41; all five behaviours reachable but Acknowledge dominates (71% on real data); Exp 41 recommends Variant B (Acknowledge exclusions via phase/progress/health gates) as the cleaner approach;
 - Decision condition status evaluation;
 - Question-to-condition relevance scoring;
 - Evidence direction classification (support, contradict, inform);
@@ -100,7 +100,7 @@ Historical documents are retained under `docs/archive/` and should be opened onl
 
 ## 8. Return-to-Work Summary
 
-Engine experiments advanced to Experiment 40, a reachability diagnostic confirming Summarise and Pause fire their rules but are always blocked by Acknowledge (priority-1 conflict), while Clarive's triggers never activate in tested scenarios due to narrow `too_broad` health conditions. All five behaviours confirmed independently reachable synthetically. No rules or engine behaviour changed. Knowledge-management phase is complete: all seven criteria met. No backlog items deleted or promoted.
+Engine experiments advanced to Experiment 41, which compared two passive alternatives for reducing Acknowledge dominance. Variant B (Acknowledge exclusions via phase/progress/health gates) is recommended over Variant A (priority reordering with false-positive side effects). Both variants converge on the same two genuine changes: `concluding → summarise` and `stalled → pause`. No production code changed.
 
 First document to read: `docs/current-project-state.md`. Then consult `.claude/architecture-guardrails.md` before any code changes and `docs/project-knowledge-inventory.md` for task-specific references. The full experiment history remains available in `docs/design-evolution-log.md` but is no longer default reading.