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/banner.test.ts

@@ -12,6 +12,12 @@ describe('linkify', () => {
     expect(linkify('[x](ftp://evil)')).toBe('x');
     expect(linkify('[y](javascript:boom)')).toBe('y');
   });
+  it('keeps root-relative links and renders several in one string', () => {
+    expect(linkify('go [home](/lobby) or [docs](https://x.com)')).toBe(
+      'go <a href="/lobby" target="_blank" rel="noopener noreferrer">home</a> or ' +
+        '<a href="https://x.com" target="_blank" rel="noopener noreferrer">docs</a>',
+    );
+  });
 });
 
 describe('banner rotator', () => {
@@ -43,4 +49,42 @@ describe('banner rotator', () => {
     expect(scrolled).toBe(1);
     r.stop();
   });
+
+  it('repeats the scroll cycle while under holdMs, then advances', () => {
+    vi.useFakeTimers();
+    const cfg = { holdMs: 2500, edgePauseMs: 100, fadeMs: 10, scrollPxPerSec: 100 };
+    const shown: number[] = [];
+    let scrolls = 0;
+    const r = createBannerRotator(
+      [{ md: 'long' }, { md: 'short' }],
+      { overflowPx: (i) => (i === 0 ? 100 : 0), show: (i) => shown.push(i), scrollTo: () => scrolls++ },
+      cfg,
+    );
+    r.start();
+    vi.advanceTimersByTime(110); // fade(10) + edgePause(100) -> first scroll
+    expect(scrolls).toBe(1);
+    expect(shown).toEqual([0]);
+    vi.advanceTimersByTime(1200); // scrollDur(1000) + edgePause + edgePause -> re-show + second scroll
+    expect(scrolls).toBe(2);
+    expect(shown).toEqual([0, 0]); // re-shown to reset scroll, still item 0 (under holdMs)
+    vi.advanceTimersByTime(10_000);
+    expect(shown).toContain(1); // eventually exceeds holdMs and advances to the fitting message
+    r.stop();
+  });
+
+  it('stop() halts further advancement', () => {
+    vi.useFakeTimers();
+    const cfg = { ...defaultBannerConfig, holdMs: 100, fadeMs: 5, edgePauseMs: 5 };
+    const shown: number[] = [];
+    const r = createBannerRotator(
+      [{ md: 'a' }, { md: 'b' }],
+      { overflowPx: () => 0, show: (i) => shown.push(i), scrollTo: () => {} },
+      cfg,
+    );
+    r.start();
+    vi.advanceTimersByTime(cfg.fadeMs);
+    r.stop();
+    vi.advanceTimersByTime(10_000);
+    expect(shown).toEqual([0]);
+  });
 });