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/us20-matrix-fits-modal.spec.ts

@@ -176,4 +176,60 @@ test.describe('US-20 — MITRE matrix fits modal', () => {
 
     await deleteSimulation(redteamToken, sim.id);
   });
+
+  // NIT code-reviewer + AC-15.5 regression: Tab focus-trap cycle in MitreMatrixModal
+  test('AC-15.5 regression — MitreMatrixModal Tab key cycles focus, Shift+Tab reverses', async ({
+    page,
+    context,
+  }) => {
+    const sim = await createSimulation(redteamToken, engagementId, 'AC-15.5 focus trap');
+
+    await seedTokenInStorage(context, redteamToken);
+    await page.goto(`/engagements/${engagementId}/simulations/${sim.id}/edit`);
+
+    await page.getByLabel(/open mitre matrix/i).click();
+    const dialog = page.getByRole('dialog');
+    await expect(dialog).toBeVisible({ timeout: 10_000 });
+
+    // Collect all focusable elements in the dialog
+    const focusableCount = await dialog.evaluate((el) => {
+      const focusables = el.querySelectorAll(
+        'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])'
+      );
+      return focusables.length;
+    });
+    expect(focusableCount).toBeGreaterThan(1);
+
+    // Focus the last focusable element
+    await dialog.evaluate((el) => {
+      const focusables = Array.from(el.querySelectorAll<HTMLElement>(
+        'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])'
+      ));
+      focusables[focusables.length - 1].focus();
+    });
+
+    // Tab from last → should wrap to first (focus trap)
+    await page.keyboard.press('Tab');
+    const activeAfterTab = await dialog.evaluate((el) => {
+      const focusables = Array.from(el.querySelectorAll<HTMLElement>(
+        'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])'
+      ));
+      return focusables.indexOf(document.activeElement as HTMLElement);
+    });
+    // After Tab from last, focus should be on first (index 0)
+    expect(activeAfterTab).toBe(0);
+
+    // Shift+Tab from first → should wrap to last
+    await page.keyboard.press('Shift+Tab');
+    const activeAfterShiftTab = await dialog.evaluate((el) => {
+      const focusables = Array.from(el.querySelectorAll<HTMLElement>(
+        'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])'
+      ));
+      return focusables.indexOf(document.activeElement as HTMLElement);
+    });
+    // After Shift+Tab from first, focus should be on last
+    expect(activeAfterShiftTab).toBe(focusableCount - 1);
+
+    await deleteSimulation(redteamToken, sim.id);
+  });
 });