docs(confidence-engine): checkpoint plausible zero-state verification

docs/current-handoff.md

@@ -357,9 +357,36 @@ Live rendered-product verification via Playwright established that one actual in
 - No production code or test changes during verification.
 - **Evidence boundary:** establishes the implemented three-part boundary in the deterministic apparatus and the one observed rendered investigation. Does not generalise to universally solved, proven across all investigations, or production-complete.
 
-**Unresolved — `Update report` placement/ownership:** OPEN PRODUCT QUESTION. A prescribed locator (`page.getByRole("button", { name: "Update report" })`) failed during verification, but this is a separate UI-ownership question (the control exists on the investigation detail page; its presence on the report page was not decided). The three-part report semantic/rendering increment is VERIFIED regardless.
+**Update report placement/ownership (RESOLVED by Iteration 4):** `Report page + only when report is STALE`. The control exists on the report page and is invoked by the user when the lifecycle shows stale. This is established production behaviour, not an open UX decision.
 
-**Report lifecycle generation/reuse:** not re-established by this browser run (zero-model-call budget). Existing lifecycle evidence remains authoritative unless separately retested. The inference that the report is "dynamically generated on page load" is not preserved from the verification record.
+**Iterated Four — Zero-Plausible Interpretations Checkpoint (2026-10-02):**
+
+Implementation commit: `0cc9d08` — "permit empty plausibleInterpretations for honest zero state"
+
+Contract properties:
+- `plausibleInterpretations` is a required string field;
+- empty string (`""`) represents a genuine zero-plausible state;
+- non-empty string represents a plausible narrative;
+- `hasPlausibleInterpretations` derives from validated output content (no hard-coding);
+- the model is explicitly permitted/required to return `""` when SECTION B is empty;
+- zero-plausible reports omit "What remains plausible";
+- non-zero behaviour remains deterministically covered.
+
+Deterministic verification: 71/71 PASS.
+
+Report lifecycle (established):
+- absent → generate on first visit;
+- current → reuse;
+- investigation changes → stale;
+- stale → user-owned Update report action;
+- no automatic regeneration.
+
+Fixed live investigation (`eae84f2d-acd8-4a92-bd43-347628b536a7`):
+- precondition: `investigationRevision = 2`, `generatedFromRevision = 1`, report = STALE, plausible inputs = 0;
+- exactly one live Update report action produced `plausibleInterpretations = ""`, `hasPlausibleInterpretations = false`, `investigationRevision = 2`, `generatedFromRevision = 2`, report = CURRENT;
+- rendered result: "What we understand" visible, "What remains plausible" omitted, "What remains unresolved" visible, obsolete absence prose absent, canonical unresolved questions preserved.
+
+**Evidence limitation:** This is specific to the one fixed zero-plausible regression case described above. The model correctly represented zero interpretations as empty string and rendered the report without a plausible section in this verified scenario. It does NOT prove universality across all scenario shapes or models.
 
 **Next step:** product-level review of the Investigation Report experience — do not automatically change implementation.
 

docs/current-project-state.md

@@ -70,6 +70,19 @@ One actual Investigation Report demonstrated three distinct epistemic sections:
 
 **Next:** product-level review of Investigation Report experience.
 
+### Iteration 4 — Zero-Plausible Interpretations Checkpoint (2026-10-02)
+
+Implementation commit: `0cc9d08` — "permit empty plausibleInterpretations for honest zero state"
+
+The fixed zero-plausible regression case established:
+- Canonical representation: `plausibleInterpretations = ""`, `hasPlausibleInterpretations = false`;
+- Deterministic verification: 71/71 PASS;
+- One live Update report call on investigation `eae84f2d` returned canonical zero state, moved report to CURRENT;
+- Report rendered correctly: "What remains plausible" omitted, "What we understand" and "What remains unresolved" intact, obsolete absence prose absent;
+- Update report ownership: Report page + only when stale (production behaviour).
+
+Evidence is specific to this one fixed live regression case. Does not generalise universally.
+
 #### Loop 2 (final semantic run — 2/2 valid calls)
 
 Observed: