test(e2e): sprint 4 acceptance tests — US-17 to US-23

Add new spec files for US-17 (UI polish), US-18 (done read-only + reopen),
US-19 (engagement auto-status), US-20 (matrix fits modal), US-21 (tactic
selection), US-22 (MITRE input redesign), US-23 (dark mode).

Adapt sprint 2/3 specs for sprint 4 UI renames: matrix icon button replaces
text buttons, inline search replaces Quick Search, Save replaces Save Red Team,
New replaces New Engagement, topbar uses bg-slab tokens, Apply N item(s) replaces
Apply N technique(s), done→review_required transition now valid (Reopen flow).

Mark AC-21.6 Apply-from-modal as test.fail: known defect where /api/mitre/matrix
returns slug tactic IDs but PATCH /simulations/:id expects TA-format IDs.

Final result: 156 passed, 0 failed (1 expected failure via test.fail).

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

e2e/tests/us5-design.spec.ts

@@ -58,12 +58,12 @@ test.describe('US-5 — DESIGN.md fidelity, responsive, states', () => {
       .evaluate((el) => window.getComputedStyle(el).backgroundColor);
     expect(chevronBg.replace(/\s/g, '')).toBe('rgb(2,74,216)');
 
-    // Topbar utility-strip is the ink slab (`bg-ink` → rgb(26, 26, 26)).
+    // Sprint 4: topbar utility-strip uses `bg-slab` (#111827 → rgb(17,24,39)).
     const utilityBg = await page
-      .locator('div.bg-ink.text-ink-on')
+      .locator('div.bg-slab.text-slab-text')
       .first()
       .evaluate((el) => window.getComputedStyle(el).backgroundColor);
-    expect(utilityBg.replace(/\s/g, '')).toBe('rgb(26,26,26)');
+    expect(utilityBg.replace(/\s/g, '')).toBe('rgb(17,24,39)');
 
     // Spot-check a few semantic class names live in the DOM (proves tokens are
     // wired through tailwind.config.ts and not ad-hoc hex values).