Stage 7: regression tests for the polished UI (logic + behaviour)

Lock the polish branch's behaviour so a future UI edit surfaces as a failing
assertion to re-agree or fix.

Unit (vitest, node env):
- placement: recallIndex, cellOccupied/isBlankSlot, non-linear direction, the
  single-tile submit default, and placementFromHint blank-fallback / rack-exhausted.
- banner: the marquee scroll-cycle repeat-then-advance, stop(), root-relative and
  multiple links.
- client.GatewayError. Extract the check-word constraints out of Game.svelte into a
  pure lib/checkword.ts (sanitize + canCheck) and cover them.

E2E (playwright mock, Chromium + WebKit):
- commit via the 🏁 control, history slide-down + close, the exchange dialog,
  check-word input sanitising + verdict, resign-to-finished, and the Settings
  board-label mode changing the on-board labels.

ui/src/lib/placement.test.ts

@@ -1,12 +1,15 @@
 import { describe, expect, it } from 'vitest';
 import {
   BLANK,
+  cellOccupied,
   direction,
+  isBlankSlot,
   newPlacement,
   place,
   placementFromHint,
   rackView,
   recallAt,
+  recallIndex,
   reset,
   toSubmit,
 } from './placement';
@@ -62,6 +65,29 @@ describe('placement state machine', () => {
     expect(toSubmit(place(newPlacement(rack), 0, 7, 7), 'V')?.dir).toBe('V');
     expect(toSubmit(newPlacement(rack))).toBeNull();
   });
+
+  it('recalls a tile by rack index and reports occupied cells / blank slots', () => {
+    let p = place(newPlacement(rack), 0, 7, 7);
+    p = place(p, 1, 7, 8);
+    expect(cellOccupied(p, 7, 7)).toBe(true);
+    expect(cellOccupied(p, 6, 6)).toBe(false);
+    p = recallIndex(p, 0);
+    expect(p.pending.map((t) => t.rackIndex)).toEqual([1]);
+    expect(isBlankSlot(newPlacement(rack), 2)).toBe(true); // '?' slot
+    expect(isBlankSlot(newPlacement(rack), 0)).toBe(false);
+  });
+
+  it('treats a non-linear placement as no inferred direction', () => {
+    let p = place(newPlacement(rack), 0, 7, 7);
+    p = place(p, 1, 8, 8); // diagonal
+    expect(direction(p)).toBeNull();
+  });
+
+  it('defaults a single-tile submit to H without an override', () => {
+    const sub = toSubmit(place(newPlacement(rack), 0, 7, 7));
+    expect(sub?.dir).toBe('H');
+    expect(sub?.tiles).toHaveLength(1);
+  });
 });
 
 describe('placementFromHint', () => {
@@ -78,4 +104,21 @@ describe('placementFromHint', () => {
     expect(p.pending[0]).toMatchObject({ rackIndex: 0, letter: 'C', blank: false });
     expect(p.pending[2]).toMatchObject({ rackIndex: 2, letter: 'B', blank: true });
   });
+
+  it('falls back to a blank slot when the hint letter is not in the rack', () => {
+    const p = placementFromHint([{ row: 7, col: 7, letter: 'Z', blank: false }], ['A', BLANK]);
+    expect(p.pending).toHaveLength(1);
+    expect(p.pending[0]).toMatchObject({ rackIndex: 1, letter: 'Z', blank: true });
+  });
+
+  it('skips hint tiles once the rack is exhausted', () => {
+    const p = placementFromHint(
+      [
+        { row: 7, col: 7, letter: 'A', blank: false },
+        { row: 7, col: 8, letter: 'B', blank: false },
+      ],
+      ['A'],
+    );
+    expect(p.pending.map((t) => t.letter)).toEqual(['A']);
+  });
 });