feat(frontend): sprint 3 — multi-technique MITRE selection + matrix modal

- types: replace mitre_technique_id/name scalars with techniques:MitreTechnique[]
  on Simulation; add MitreTactic/MitreMatrixTechnique/MitreMatrixSubtechnique;
  SimulationPatchInput now uses technique_ids:string[]
- api/mitre.ts: add getMitreMatrix() → GET /api/mitre/matrix
- hooks/useMitre: add useMitreMatrix(enabled) with staleTime:Infinity
- MitreTechniquePicker: clean rewrite — onSelect(technique) one-shot, resets
  input after selection, no incoming value props
- MitreTechniqueTag: chip component with id+name and × remove button
- MitreMatrixModal: tactic columns (220px fixed), expand/collapse subtechniques,
  search filter (auto-expands parent on sub match), selection state, focus trap
  (Tab wrap, Escape, search autofocus), backdrop click cancel, Apply N techniques
- MitreTechniquesField: orchestrates tags+picker+matrix with auto-save PATCH on
  every add/remove/Apply, dedup guard, disabled read-only mode for SOC
- SimulationFormPage: swap MitreTechniquePicker for MitreTechniquesField; remove
  technique state from RT form (techniques have independent auto-save cycle)
- SimulationList: MITRE column → T1059 +2 counter format, — when empty
- Tests: 84 passing (13 test files); new suites for Tag, Field, Modal;
  MitreTechniquePicker + SimulationFormPage + SimulationList adapted to new API

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

frontend/tests/MitreMatrixModal.test.tsx

@@ -0,0 +1,206 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import MockAdapter from 'axios-mock-adapter';
+import { apiClient } from '@/api/client';
+import { MitreMatrixModal } from '@/components/MitreMatrixModal';
+import { renderWithProviders } from './utils';
+import type { MitreTactic, MitreTechnique } from '@/api/types';
+
+const MATRIX: MitreTactic[] = [
+  {
+    tactic_id: 'TA0001',
+    tactic_name: 'Initial Access',
+    techniques: [
+      {
+        id: 'T1078',
+        name: 'Valid Accounts',
+        subtechniques: [
+          { id: 'T1078.001', name: 'Default Accounts' },
+          { id: 'T1078.002', name: 'Domain Accounts' },
+        ],
+      },
+      {
+        id: 'T1190',
+        name: 'Exploit Public-Facing Application',
+        subtechniques: [],
+      },
+    ],
+  },
+  {
+    tactic_id: 'TA0002',
+    tactic_name: 'Execution',
+    techniques: [
+      {
+        id: 'T1059',
+        name: 'Command and Scripting Interpreter',
+        subtechniques: [{ id: 'T1059.001', name: 'PowerShell' }],
+      },
+    ],
+  },
+];
+
+const SELECTION: MitreTechnique[] = [
+  { id: 'T1078', name: 'Valid Accounts', tactics: ['initial-access'] },
+];
+
+describe('MitreMatrixModal', () => {
+  let mock: MockAdapter;
+
+  beforeEach(() => {
+    mock = new MockAdapter(apiClient);
+    mock.onGet('/mitre/matrix').reply(200, MATRIX);
+  });
+
+  afterEach(() => {
+    mock.restore();
+  });
+
+  it('returns null when isOpen=false', () => {
+    const { container } = renderWithProviders(
+      <MitreMatrixModal isOpen={false} initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+    expect(container.firstChild).toBeNull();
+  });
+
+  it('renders dialog with tactic columns when open', async () => {
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+    await waitFor(() => {
+      expect(screen.getByText('Initial Access')).toBeInTheDocument();
+      expect(screen.getByText('Execution')).toBeInTheDocument();
+    });
+  });
+
+  it('renders techniques for each tactic', async () => {
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+    await waitFor(() => {
+      expect(screen.getByText('T1078')).toBeInTheDocument();
+      expect(screen.getByText('T1059')).toBeInTheDocument();
+    });
+  });
+
+  it('Apply button calls onApply with selected techniques', async () => {
+    const onApply = vi.fn();
+    const user = userEvent.setup();
+
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={onApply} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // Click the label button for T1078 to select it
+    const t1078Btn = screen.getAllByRole('button').find(
+      (btn) => btn.textContent?.includes('T1078') && !btn.getAttribute('aria-label'),
+    );
+    await user.click(t1078Btn!);
+
+    await user.click(screen.getByRole('button', { name: /Apply/i }));
+
+    expect(onApply).toHaveBeenCalledWith(
+      expect.arrayContaining([expect.objectContaining({ id: 'T1078' })]),
+    );
+  });
+
+  it('Cancel button calls onCancel without onApply', async () => {
+    const onApply = vi.fn();
+    const onCancel = vi.fn();
+    const user = userEvent.setup();
+
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={onApply} onCancel={onCancel} />,
+    );
+
+    await user.click(screen.getByRole('button', { name: /Cancel/i }));
+
+    expect(onCancel).toHaveBeenCalled();
+    expect(onApply).not.toHaveBeenCalled();
+  });
+
+  it('Escape key calls onCancel', async () => {
+    const onCancel = vi.fn();
+    const user = userEvent.setup();
+
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={onCancel} />,
+    );
+
+    await user.keyboard('{Escape}');
+
+    expect(onCancel).toHaveBeenCalled();
+  });
+
+  it('shows initial selection as selected', async () => {
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={SELECTION} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // T1078 should show selected count in tactic header
+    expect(screen.getByText('1 selected')).toBeInTheDocument();
+  });
+
+  it('search filter narrows visible techniques', async () => {
+    const user = userEvent.setup();
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    const searchInput = screen.getByPlaceholderText(/Filter techniques/i);
+    await user.type(searchInput, 'T1059');
+
+    // T1059 column should be visible, T1078 should not
+    expect(screen.queryByText('T1078')).toBeNull();
+    expect(screen.getByText('T1059')).toBeInTheDocument();
+  });
+
+  it('chevron expands subtechniques', async () => {
+    const user = userEvent.setup();
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // Subtechniques should not be visible initially
+    expect(screen.queryByText(/Default Accounts/)).toBeNull();
+
+    // Click the expand chevron for T1078
+    const expandBtn = screen.getByRole('button', { name: /Expand T1078/i });
+    await user.click(expandBtn);
+
+    expect(screen.getByText(/Default Accounts/)).toBeInTheDocument();
+  });
+
+  it('Apply button shows technique count', async () => {
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={SELECTION} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByRole('button', { name: /Apply 1 technique/i })).toBeInTheDocument();
+    });
+  });
+
+  it('backdrop click calls onCancel', async () => {
+    const onCancel = vi.fn();
+    const user = userEvent.setup();
+
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={onCancel} />,
+    );
+
+    // Click the backdrop (the fixed inset div behind the modal)
+    const backdrop = document.querySelector('.bg-ink\\/60') as HTMLElement;
+    if (backdrop) await user.click(backdrop);
+
+    expect(onCancel).toHaveBeenCalled();
+  });
+});