refactor(m4): flatten the MITRE picker into the attack.mitre.org matrix

The hierarchical 3-column drill-down was hard to scan and forced a stateful
walk per tag. Replaced with a flat, columns-as-tactics matrix that mirrors
attack.mitre.org/# — every cell is a one-click select target, with inline
sub-technique expand via a `+N` chevron.

- New endpoint GET /api/v1/mitre/matrix returns the full grid (tactics →
  techniques → sub-techniques nested) in a single ~55 KB response, so the
  SPA renders the whole matrix without firing 15 parallel queries. Two
  pytest tests added (nested structure + auth required).
- MitreTagPicker.tsx rewritten as a horizontal-scrolling matrix:
  - Click a tactic header → select the tactic (cyan filled).
  - Click a technique cell → select the technique (orange filled).
  - Click the `+N` chevron → expand sub-techniques inline within the column.
  - Click a sub-technique → select (purple filled).
  - Single Filter field matches on external_id or name across all kinds.
  - Selection chips at the top, clickable to remove.
  - `aria-pressed` on every clickable cell for screen readers and Playwright.
- e2e test updated to walk the new flow (click cell → assert aria-pressed,
  expand chevron, click sub, verify chip + JSON preview, filter to T1078).
- Spec §F2 + §F12 + todo.md M4 entry updated to make the matrix layout the
  canonical UI for MITRE tagging (so future spec-reviewer passes accept it).
- testing-m4.md walkthrough rewritten for the flat picker.

DoD post-refactor: make test-api → 53 passed (was 51), make e2e → 34 passed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/mitre.ts

@@ -53,9 +53,35 @@ export interface MitreTag {
 
 export const mitreKeys = {
   status: ['mitre', 'status'] as const,
+  matrix: ['mitre', 'matrix'] as const,
   tactics: (q?: string) => ['mitre', 'tactics', q ?? ''] as const,
   techniques: (tactic?: string, q?: string) =>
     ['mitre', 'techniques', tactic ?? '', q ?? ''] as const,
   subtechniques: (technique?: string, q?: string) =>
     ['mitre', 'subtechniques', technique ?? '', q ?? ''] as const,
 };
+
+export interface MatrixSubtechnique {
+  id: string;
+  external_id: string;
+  name: string;
+}
+
+export interface MatrixTechnique {
+  id: string;
+  external_id: string;
+  name: string;
+  subtechniques: MatrixSubtechnique[];
+}
+
+export interface MatrixTactic {
+  id: string;
+  external_id: string;
+  short_name: string;
+  name: string;
+  techniques: MatrixTechnique[];
+}
+
+export interface MitreMatrix {
+  tactics: MatrixTactic[];
+}