feat(inter-agent): surface frontend des annonces live sur les cellules (F1-F3)

Affiche les annonces d'agent en direct au-dessus des cellules, sur la base
develop :

- announcements: nouveau feature module — store réactif, provider
  d'abonnement aux événements, overlay par cible et aperçu, avec tests
  (announcementsStore + provider).
- App.tsx: montage du provider dans l'arbre applicatif.
- domain/index.ts: types partagés de l'événement d'annonce côté front.
- layout: composition de l'overlay dans LayoutGrid + règle d'exclusion
  couverte par overlayExclusion.test.ts.
- AgentsPanel: intègre l'aperçu des annonces dans la surface existante.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/overlayExclusion.test.ts

@@ -0,0 +1,58 @@
+/**
+ * Ticket #4 — mutual exclusion of the two full-cell veils in a `LeafView`
+ * (Architect arbitrage `ticket4-overlay-composition-leafview`).
+ *
+ * `shouldShowWritePortalVeil` is the exact predicate the JSX uses to gate the
+ * write-portal veil; the F3 target overlay self-gates on the *same* `busyActive`
+ * source (`useTargetAnnouncements(agentId).active`). Testing this predicate
+ * therefore proves the on-screen invariant: exactly one veil at a time, F3 first.
+ */
+
+import { describe, it, expect } from "vitest";
+
+import { shouldShowWritePortalVeil } from "./LayoutGrid";
+
+/** Mirrors the F3 overlay's own render guard (`active` ⇔ busy). */
+const showsF3 = (agentPinned: boolean, busyActive: boolean) =>
+  agentPinned && busyActive;
+
+describe("shouldShowWritePortalVeil — veil exclusion", () => {
+  it("hides the write-portal veil while F3 is active (busy), whatever the portal says", () => {
+    // The nominal inter-agent injection path: overlay AND busy rise together.
+    expect(shouldShowWritePortalVeil(true, true, true)).toBe(false);
+    // Busy without a portal request: still no write-portal veil.
+    expect(shouldShowWritePortalVeil(true, false, true)).toBe(false);
+  });
+
+  it("shows the write-portal veil as the fallback when busy is not (yet) set", () => {
+    expect(shouldShowWritePortalVeil(true, true, false)).toBe(true);
+  });
+
+  it("shows no write-portal veil when the portal is idle", () => {
+    expect(shouldShowWritePortalVeil(true, false, false)).toBe(false);
+  });
+
+  it("never shows the write-portal veil for a plain (agent-less) cell", () => {
+    expect(shouldShowWritePortalVeil(false, true, true)).toBe(false);
+    expect(shouldShowWritePortalVeil(false, true, false)).toBe(false);
+  });
+
+  it("NEVER renders both veils simultaneously (exhaustive truth table)", () => {
+    for (const agentPinned of [false, true]) {
+      for (const overlay of [false, true]) {
+        for (const busyActive of [false, true]) {
+          const writePortal = shouldShowWritePortalVeil(
+            agentPinned,
+            overlay,
+            busyActive,
+          );
+          const f3 = showsF3(agentPinned, busyActive);
+          // The safety invariant: the two full-cell veils are mutually exclusive.
+          expect(writePortal && f3).toBe(false);
+          // And F3 has priority: whenever it is up, the write-portal veil is down.
+          if (f3) expect(writePortal).toBe(false);
+        }
+      }
+    }
+  });
+});