feat(confidence-engine): v0.60f allocate investigation identity on create

Replace Portfolio's static "+ Create new investigation" link (href:
/investigations/case-1) with a <button> that allocates an opaque
application-owned durable ID via crypto.randomUUID() and navigates
via router.push to /investigations/{id} without persisting any empty
Investigation.

INVESTIGATION_ID constant retained only for card links (Continue
investigation / View report) — not migrated in this increment.

Test: deterministic Create New activation test verifies UUID allocation,
navigation to generated ID route, and zero saveInvestigation calls.

app/page.jsx

@@ -3,10 +3,12 @@
 import React from "react";
 import { loadInvestigation, clearInvestigation } from "@/lib/storage/investigation-storage";
 import Link from "next/link";
+import { useRouter } from "next/navigation";
 
 const INVESTIGATION_ID = "case-1";
 
 function Portfolio() {
+  const router = useRouter();
   const [existing, setExisting] = React.useState(null);
   const [showRestartConfirm, setShowRestartConfirm] = React.useState(false);
 
@@ -132,12 +134,16 @@ function Portfolio() {
         </section>
       )}
 
-      <Link
-        href={`/investigations/${INVESTIGATION_ID}`}
+      <button
+        onClick={(e) => {
+          e.preventDefault();
+          const id = crypto.randomUUID();
+          router.push(`/investigations/${id}`);
+        }}
         className="rounded-lg border-[2.5px] border-dashed border-teal-400 px-6 py-3 text-sm font-medium text-teal-700 hover:bg-teal-50 transition"
       >
         + Create new investigation
-      </Link>
+      </button>
     </main>
   );
 }