feat(confidence-engine): case/update synthesis — dedicated reconstruction per update (v0.50)

Architecture: after successful /api/cases/update, derive explicit nextGraph +
nextFindings, call synthesizeFromFindings exactly once, replace Current
Understanding with reconstruction result.

Key invariants:
- outcome.summary retired as final CU authority → always synthesis reconstruction
- Explicit derived state (no React-state reread) for graph and findings
- Previous CU preserved on synthesis failure (no fallback to outcome.summary)
- Graph and Findings NOT lost on synthesis failure
- saveInvestigation persistence uses currentUnderstanding, not outcome.summary

Deterministic regression: 7 tests (Cases A-E + 2 edges) covering all rules.

Files: components/scenario-form.jsx, tests/ui/scenario-form-case-update-synthesis.test.jsx

components/scenario-form.jsx

@@ -577,23 +577,21 @@ export default function ScenarioForm() {
       const outcome = submission.data;
 
       if (submission.ok && outcome.success) {
-        // Merge server-returned findings with local state
-        let newFindings = [...findings];
+        // ── Derive explicit next canonical state (no React-state reread) ──
+        const nextGraph = outcome.updatedSituationGraph;
+        let nextFindings = [...findings];
         if (outcome.appendedFindings && Array.isArray(outcome.appendedFindings)) {
-          newFindings = [...newFindings, ...outcome.appendedFindings];
+          nextFindings = [...nextFindings, ...outcome.appendedFindings];
         }
 
         setUpdateStatus("success");
-        setCurrentUnderstanding(
-          outcome.summary ? outcome.summary : currentUnderstanding,
-        );
         setUpdateResult({
           ...outcome,
           previousSituationGraph: result?.situationGraph ?? null,
         });
         setResult((current) => ({
           ...current,
-          situationGraph: outcome.updatedSituationGraph,
+          situationGraph: nextGraph,
           selectedQuestion: normaliseUpdateSelectedQuestion(
             outcome.selectedQuestion,
           ),
@@ -602,9 +600,22 @@ export default function ScenarioForm() {
             .map((node) => node.id),
           diagnostics: outcome.diagnostics,
         }));
+        setFindings(nextFindings);
+
+        // ── Coalesced transition: one synthesis per successful update ──
+        void synthesizeFromFindings(fetch, {
+          situationGraph: nextGraph,
+          findings: normalizeFindings(nextFindings),
+        }).then((res) => {
+          if (res.ok && res.data?.currentUnderstanding) {
+            setCurrentUnderstanding(res.data.currentUnderstanding);
+          }
+          // On synthesis failure: graph/Findings already persisted, CU preserved, no retry.
+        });
+
         setAnswer("");
-        // Persist after successful update turn — include findings
-        saveInvestigation({ scenario, situationGraph: outcome.updatedSituationGraph, selectedQuestion: normaliseUpdateSelectedQuestion(outcome.selectedQuestion), summary: outcome.summary ?? currentUnderstanding, updatedAt: new Date().toISOString(), focusedContributions, findings: newFindings });
+        // Persist after successful update turn — include explicit next state
+        saveInvestigation({ scenario, situationGraph: nextGraph, selectedQuestion: normaliseUpdateSelectedQuestion(outcome.selectedQuestion), summary: currentUnderstanding, updatedAt: new Date().toISOString(), focusedContributions, findings: nextFindings });
       } else {
         setUpdateStatus("error");
         setUpdateError(outcome);