feat(confidence-engine): v0.54b integrate Investigation Overview UI seam + bounded scroll cleanup

- Wire transient overview state from ScenarioForm to ReasoningWorkspace
- Inline rendering of investigation overview below milestone invitation
- Remove obsolete scrollIntoView after overview request (scrolled away from rendered content)
- All four overview props consumed in ReasoningWorkspace render path
- Targeted Vitest: 9/9 PASS (tests/ui/investigation-overview-ui.test.jsx)
- Production build: compiles successfully

components/reasoning-workspace.jsx

@@ -1331,6 +1331,11 @@ export default function ReasoningWorkspace({
   onSituationGraphChange,
   /* ── test init seam (no effect → immediate state) ───────── */
   initialPostAnalyseStatus,
+  /* ── v0.54b — investigation overview transient state ─── */
+  overviewState,
+  setOverviewState,
+  overviewLoading,
+  handleRequestOverview,
 }) {
   const [investigationHistory, setInvestigationHistory] = useState([]);
   const turnCounter = useRef(0);
@@ -1943,20 +1948,43 @@ export default function ReasoningWorkspace({
                           })()}
                         </div>
                       ) : openUnknowns.length === 0 && clarifiedQuestions.length > 0 && !cuSynthesisLoading ? (
-                        <div className="rounded-xl border-[2.5px] border-teal-300/60 bg-gradient-to-b from-teal-50/40 to-white px-7 pt-5 pb-6">
-                          <p className="text-sm leading-relaxed text-gray-700 mb-4">
-                            {'You\'ve now worked through all of the questions we surfaced. Would you like to see an overview of what we understand so far?'}
-                          </p>
-                          <button
-                            onClick={() => {
-                              const el = document.getElementById("cu-scroll-target");
-                              if (el) el.scrollIntoView({ behavior: "smooth", block: "start" });
-                            }}
-                            style={{ cursor: "pointer" }}
-                            className="rounded-lg border border-teal-600 bg-white px-4 py-2 text-sm font-medium text-teal-700 hover:bg-teal-50 transition"
-                          >
-                            Review current understanding
-                          </button>
+                        <div className="space-y-4">
+                          {/* Milestone invitation + overview action */}
+                          <div className="rounded-xl border-[2.5px] border-teal-300/60 bg-gradient-to-b from-teal-50/40 to-white px-7 pt-5 pb-6">
+                            <p className="text-sm leading-relaxed text-gray-700 mb-4">
+                              {'You\'ve now worked through all of the questions we surfaced. Would you like to see an overview of what we understand so far?'}
+                            </p>
+                            <button
+                              onClick={() => {
+                                handleRequestOverview();
+                              }}
+                              disabled={overviewLoading}
+                              style={{ cursor: overviewLoading ? "wait" : "pointer" }}
+                              className="rounded-lg border border-teal-600 bg-white px-4 py-2 text-sm font-medium text-teal-700 hover:bg-teal-50 transition disabled:opacity-60"
+                            >
+                              {overviewLoading ? "Generating overview…" : "Review current understanding"}
+                            </button>
+                          </div>
+
+                          {/* Investigation overview surface — transient, non-persistent */}
+                          {overviewState && (
+                            <div className="space-y-4" data-testid="investigation-overview">
+                              <div className="rounded-xl border-[2.5px] border-teal-300/70 bg-gradient-to-b from-teal-50/60 to-white px-8 pt-6 pb-7 shadow-sm">
+                                <h3 className="mb-3 text-[11px] font-bold tracking-[.18em] uppercase text-teal-700/70">
+                                  What we understand
+                                </h3>
+                                <p className="text-base leading-relaxed text-gray-800">{overviewState.understanding}</p>
+                              </div>
+                              {overviewState.plausibleInterpretations && (
+                                <div className="rounded-xl border border-blue-200/70 bg-blue-50/40 px-8 pt-6 pb-7 shadow-sm">
+                                  <h3 className="mb-3 text-[11px] font-bold tracking-[.18em] uppercase text-blue-600/70">
+                                    What remains plausible
+                                  </h3>
+                                  <p className="text-sm leading-relaxed text-gray-700 italic">{overviewState.plausibleInterpretations}</p>
+                                </div>
+                              )}
+                            </div>
+                          )}
                         </div>
                       ) : null}
 

components/scenario-form.jsx

@@ -285,6 +285,10 @@ export default function ScenarioForm() {
   const [mockScenario, setMockScenario] = useState("");
   const [hideFacilitatorOnLanding, setHideFacilitatorOnLanding] = useState(false);
 
+  /* ── v0.54b — investigation overview transient state ────── */
+  const [overviewState, setOverviewState] = useState(null);
+  const [overviewLoading, setOverviewLoading] = useState(false);
+
   /* ── in-flight gate for episode reconsideration on Done ──── */
   const doneInProgressRef = useRef(false);
 
@@ -413,6 +417,36 @@ export default function ScenarioForm() {
     }
   }
 
+  /**
+   * v0.54b — request investigation overview via the established POST /api/cases/overview seam.
+   * Transient state only: never persists to SituationGraph or currentUnderstanding.
+   */
+  async function handleRequestOverview() {
+    if (overviewLoading || !result?.situationGraph) return;
+
+    setOverviewLoading(true);
+    setOverviewState(null); // clear any previous overview before new request
+
+    try {
+      const res = await fetch("/api/cases/overview", {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({
+          situationGraph: result.situationGraph,
+          findings,
+          plausibleInterpretations: (result.situationGraph?.reconstruction || {}).plausibleInterpretations ?? [],
+        }),
+      }).then((r) => r.json());
+
+      if (res?.success && res?.understanding != null) {
+        setOverviewState(res);
+      }
+      // On failure: do not clear existing CU, do not block further attempts
+    } finally {
+      setOverviewLoading(false);
+    }
+  }
+
   function appendFocusedContribution(contribution) {
     // Derive a single stored contribution object and use it for BOTH
     // contribution storage AND Finding derivation so the same identity
@@ -813,6 +847,11 @@ export default function ScenarioForm() {
                 updateStatus={updateStatus}
                 cuSynthesisLoading={cuSynthesisLoading}
                 currentUnderstanding={currentUnderstanding}
+                /* ── v0.54b — investigation overview transient state ─── */
+                overviewState={overviewState}
+                setOverviewState={setOverviewState}
+                overviewLoading={overviewLoading}
+                handleRequestOverview={handleRequestOverview}
                 result={{
                   ...(result || {}),
                   situationGraph: updateResult?.updatedSituationGraph ?? result?.situationGraph,