feat(confidence-engine): v0.60e route investigation identity

Migrate the Investigation page route to own durable investigation identity
via its route [id] segment, passing that ID through to ScenarioForm for
hydration and persistence.

- Remove hardcoded INVESTIGATION_ID constant from page.jsx
- Use params.id as routeId; loadInvestigation(routeId) loads by identity
- Pass investigationId prop into ScenarioForm in both branch paths
- Session restore calls loadInvestigation(investigationId)
- All 4 save call sites include id: investigationId in snapshot
- Missing identified Investigation starts clean (no singleton fallback)
- Legacy singleton is not migrated/fallback-loaded
- Portfolio remains unmigrated; Report remains unmigrated; Restart untouched

Deterministic tests: 34/34 pass (scenario-form-persistence + investigation-storage)
Build: PASS
Live Playwright: all criteria verified at /investigations/v060e-live

app/investigations/[id]/page.jsx

@@ -4,18 +4,18 @@ import React from "react";
 import { loadInvestigation } from "@/lib/storage/investigation-storage";
 import ScenarioForm from "@/components/scenario-form";
 import Link from "next/link";
-import { useRouter } from "next/navigation";
+import { useParams, useRouter } from "next/navigation";
 import { useEffect, useState } from "react";
 
-const INVESTIGATION_ID = "case-1";
-
-export default function InvestigationPage() {
+export default function InvestigationPage({ params }) {
   const router = useRouter();
+  const routeId = typeof params?.id === "string" ? params.id : "";
   const [existing, setExisting] = useState(null);
 
   useEffect(() => {
-    setExisting(loadInvestigation());
-  }, []);
+    if (!routeId) return;
+    setExisting(loadInvestigation(routeId));
+  }, [routeId]);
 
   return (
     <main className="mx-auto max-w-[1600px] px-6 py-12">
@@ -37,12 +37,14 @@ export default function InvestigationPage() {
       </p>
       {existing ? (
         <ScenarioForm
+          investigationId={routeId}
           existingSnapshot={existing}
-          onNavigateToReport={() => router.push(`/investigations/${INVESTIGATION_ID}/report`)}
+          onNavigateToReport={() => router.push(`/investigations/${routeId}/report`)}
         />
       ) : (
         <ScenarioForm
-          onNavigateToReport={() => router.push(`/investigations/${INVESTIGATION_ID}/report`)}
+          investigationId={routeId}
+          onNavigateToReport={() => router.push(`/investigations/${routeId}/report`)}
         />
       )}
     </main>