test(e2e): fill coverage gaps — +N suffix + focus-trap cycle

Add two tests omitted from the initial sprint 4 run:
- us21: SimulationList MITRE column shows "TA0007 +2" for 1 tactic + 2 techniques
- us20: MitreMatrixModal Tab wraps to first focusable, Shift+Tab wraps to last

Suite: 158 passed, 0 failed (1 expected test.fail for AC-21.6 slug defect).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

e2e/tests/us21-tactic-selection.spec.ts

@@ -271,4 +271,27 @@ test.describe('US-21 — tactic selection', () => {
 
     await deleteSimulation(redteamToken, sim.id);
   });
+
+  // NIT code-reviewer: +N suffix in SimulationList MITRE column
+  test('AC-21.7 — SimulationList MITRE column shows first id + "+N" for mixed tactics+techniques', async ({
+    page,
+    context,
+  }) => {
+    const sim = await createSimulation(redteamToken, engagementId, 'AC-21.7 +N suffix');
+    await makeClient(redteamToken).patch(`/simulations/${sim.id}`, {
+      tactic_ids: ['TA0007'],
+      technique_ids: ['T1059', 'T1078'],
+    });
+
+    await seedTokenInStorage(context, redteamToken);
+    await page.goto(`/engagements/${engagementId}`);
+
+    // The row for this simulation should show "TA0007 +2" in the MITRE column
+    // (1 tactic TA0007 is first, then +2 for T1059 and T1078)
+    const simRow = page.getByRole('row').filter({ hasText: 'AC-21.7 +N suffix' });
+    await expect(simRow).toBeVisible({ timeout: 5_000 });
+    await expect(simRow).toContainText('TA0007 +2');
+
+    await deleteSimulation(redteamToken, sim.id);
+  });
 });