feat(web-ui): IdentityDetail page + conditional Identity badge on AttackerDetail

Third of the five-step identity-resolution substrate. Frontend hooks
into the empty /api/v1/identities/* surface from commit 2; renders
nothing visible when identity_id is null (which is the universal state
until the clusterer ships).

* decnet_web/src/components/IdentityDetail.tsx — new page. Header with
  uuid + optional CAMPAIGN / MERGED-INTO badges, stats row
  (observations / JA3 / HASSH / payloads / C2), fingerprint tag lists
  parsed from the JSON-in-TEXT columns, observations table that links
  back to AttackerDetail, conditional analyst-notes panel.
* decnet_web/src/components/AttackerDetail.tsx — IDENTITY badge
  inserted in the header row alongside TRAVERSAL. Clicking navigates
  to /identities/<uuid>. AttackerData interface gains the optional
  identity_id field.
* decnet_web/src/App.tsx — /identities/:id route + lazy-loaded chunk.

Verified by `tsc --noEmit` (clean) and `vite build` (clean — produces
IdentityDetail-*.js as its own lazy chunk). The repo has no JS test
harness; build + type-check are the gate.

decnet_web/src/components/AttackerDetail.tsx

@@ -46,6 +46,11 @@ interface AttackerBehavior {
 interface AttackerData {
   uuid: string;
   ip: string;
+  // Resolved identity FK. NULL while the clusterer hasn't run on this
+  // observation yet, or hasn't seen enough stable signal (JA3, HASSH,
+  // payload hash, C2 callback) to claim a same-hands match. See
+  // development/IDENTITY_RESOLUTION.md.
+  identity_id?: string | null;
   first_seen: string;
   last_seen: string;
   event_count: number;
@@ -1405,6 +1410,24 @@ const AttackerDetail: React.FC = () => {
         {attacker.is_traversal && (
           <span className="traversal-badge" style={{ fontSize: '0.8rem' }}>TRAVERSAL</span>
         )}
+        {/* Conditional Identity badge — surfaces only when the clusterer
+            has linked this observation to a resolved actor identity.
+            Zero behavior change when identity_id is null (which is
+            uniformly true until the clusterer ships). */}
+        {attacker.identity_id && (
+          <span
+            className="traversal-badge"
+            style={{
+              fontSize: '0.8rem',
+              cursor: 'pointer',
+              letterSpacing: '2px',
+            }}
+            title="Resolved identity — click to view all observations linked to this actor"
+            onClick={() => navigate(`/identities/${attacker.identity_id}`)}
+          >
+            IDENTITY · {attacker.identity_id.slice(0, 8)}
+          </span>
+        )}
       </div>
 
       {/* Stats Row */}