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/src/components/SimulationList.tsx

@@ -95,7 +95,11 @@ export function SimulationList({ engagementId }: SimulationListProps): JSX.Eleme
                   </Link>
                 </td>
                 <td className="px-xl py-md text-charcoal text-[14px]">
-                  {sim.mitre_technique_id ?? '—'}
+                  {sim.techniques.length === 0
+                    ? '—'
+                    : sim.techniques.length === 1
+                      ? sim.techniques[0].id
+                      : `${sim.techniques[0].id} +${sim.techniques.length - 1}`}
                 </td>
                 <td className="px-xl py-md">
                   <SimulationStatusBadge status={sim.status} />