fix(reasoning): enforce terminal post-mutation eligibility

docs/current-handoff.md

@@ -234,6 +234,14 @@ Experiment 54N tested whether an interpretation disagreement can be judged for m
 - Focused preservation run passed via `npx vitest run tests/graph/apply-proposal.test.js -t "60B.42|60B.11|replaces downstream pricing"`.
 - Broader duplicated post-mutation eligibility checks remain unchanged; this fixes the selector contract only, not the full stale-question lifecycle.
 
+### 60B.43 terminal post-mutation eligibility
+
+- Extended terminal-status exclusion across the remaining post-mutation eligibility checks in `lib/graph/apply-proposal.js` while leaving pre-mutation `validateSelectedQuestion(...)` unchanged.
+- `isSelectableUnresolvedUnknown(...)`, `remainingUnknownExists`, carried-active reuse, unresolved/eligible candidate listing, and pattern-compatible post-mutation selection now all treat `known|resolved|contradicted` as terminal/non-selectable.
+- Deterministic 60B.37-shaped closure regression now passes: proposal applies, `n_enterprise_customer_signing` resolves, `n_product_launch_decision` becomes known, `activeUnknownNodeId` becomes null, and final `selectedQuestion` becomes null.
+- Focused fallback case also passes: when the proposal-selected target becomes known but another real unresolved unknown remains, the terminal target is discarded and the real unresolved node is selected.
+- Focused preservation run passed via `npx vitest run tests/graph/apply-proposal.test.js -t "60B.43|60B.11|replaces downstream pricing"`; live rerun still required to prove the same outcome end-to-end under live conditions.
+
 ### When This Knowledge-Management Phase Is Complete
 
 Provisional criteria for review (all confirmed met by Experiment 38 cold-start test):

docs/experiment-60b43.md

@@ -0,0 +1,123 @@
+# Experiment 60B.43 — Terminal post-mutation eligibility
+
+**Date:** 2026-08-14
+**Branch:** `feature/post-mutation-terminal-eligibility-v0.38`
+
+## Purpose
+
+Extend the 60B.42 selector guard to the remaining post-mutation question-target eligibility checks so terminal-status unknown nodes cannot remain active or become the final selected question after mutation.
+
+## Starting point from 60B.42
+
+60B.42 fixed `selectActiveUnknownCandidate(...)` so it no longer scores or returns unknown-kind nodes whose status is terminal:
+
+- `known`
+- `resolved`
+- `contradicted`
+
+That closed one fallback source, but several independent post-mutation checks in `apply-proposal.js` still used weaker eligibility rules and could keep terminal nodes alive through other paths.
+
+## Remaining post-mutation eligibility changes
+
+This experiment changed post-mutation eligibility only in `lib/graph/apply-proposal.js`.
+
+Updated paths:
+
+- `isSelectableUnresolvedUnknown(...)`
+- `remainingUnknownExists`
+- `carriedActiveUnknownStillUnresolved`
+- `listUnresolvedUnknownCandidates(...)`
+- `listEligibleUnknownCandidates(...)`
+- `selectPatternCompatibleUnknownCandidate(...)`
+
+### Terminal-status rule applied post-mutation
+
+A selectable unresolved post-mutation target now requires:
+
+```text
+kind === unknown
+status NOT IN [known, resolved, contradicted]
+not in resolvedNodeIds
+```
+
+### Important boundary preserved
+
+`validateSelectedQuestion(...)` was **not** changed.
+
+The closure proposal remains valid pre-mutation even when:
+
+- `selectedQuestion.nodeId = n_product_launch_decision`
+- the same proposal updates `n_product_launch_decision -> known`
+
+Only after mutation is that now-terminal target discarded.
+
+## 60B.37 deterministic regression
+
+Added focused regression:
+
+- `60B.43 — terminal post-mutation target is cleared after valid decision closure`
+
+Reproduced the 60B.37-shaped closure:
+
+- existing active unknown: `n_enterprise_customer_signing`
+- proposal resolves `n_enterprise_customer_signing`
+- proposal updates `n_product_launch_decision -> known`
+- proposal still selects `n_product_launch_decision`
+- no added nodes or edges
+
+### Result
+
+- proposal applied successfully
+- customer factor resolved in place
+- decision became known in place
+- both options preserved unchanged
+- `activeUnknownNodeId = null`
+- final `selectedQuestion = null`
+
+## Fallback-to-real-unknown result
+
+Added a second focused case where:
+
+- proposal-selected target becomes `known`
+- another genuine unresolved unknown remains after mutation
+
+Result:
+
+- terminal known target discarded
+- remaining genuine unresolved candidate selected
+
+## Known-only post-mutation result
+
+Added a valid mutated case where the final remaining unknown becomes `known` in the same mutation.
+
+Result:
+
+- `activeUnknownNodeId = null`
+- `selectedQuestion = null`
+
+## Preservation checks
+
+Focused run also preserved:
+
+- 60B.11 preferred-target behaviour
+- pricing prerequisite-first behaviour
+- resolved exclusion
+- contradicted exclusion
+
+## Validation
+
+Command run:
+
+```bash
+npx vitest run tests/graph/apply-proposal.test.js -t "60B.43|60B.11|replaces downstream pricing"
+```
+
+Result:
+
+- PASS — `14 passed | 76 skipped`
+
+## What remains unproven until live rerun
+
+This deterministic bounded fix now clears the exact 60B.37-shaped stale target through the post-mutation production path under test.
+
+What remains unproven until a live rerun is whether the full runtime/orchestration path with the real customer-signing closure answer produces the same null-question closure end state under live conditions.