Keep a playing video when the window comes back to the front (#402)

Chrome draws no frames for a hidden page, so the scene stopped asking
for its videos. The sweeper of the VideoLibrary kept running and
dropped every player after 2 s. Back in the window, the video started
over, and Pixi warned that a texture source was destroyed while still
bound to a shader.

- The sweeper skips a hidden page. The time hidden does not count as
  idle, the 2 s start once the page is shown again.
- A dropped player only unloads its video source instead of destroying
  it. A shader that still holds the source no longer warns.
- CI serves videos through a new router for php -S. The built-in
  server answers no Range requests, so Chrome could not seek. e2e 66
  had 4 red checks in the suite job of d48f174 because of it. nginx on
  stein2 hid this.

Tests:
- Unit video-library.test.ts (new): 3 tests
- e2e 66 (atx): ALL PASSED, 18 checks
- Router checked with curl: 206 with the right bytes
- Counter-check: the old sweep fails both hidden-page tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/viewport/video-library.test.ts

@@ -0,0 +1,86 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+/**
+ * When the video library lets a player go (#402): after 2 s nobody asked for it, but never while
+ * the page is hidden. Chrome draws no frames for a hidden page and so nothing asks, while the
+ * sweeper's timer goes on; a sprite still wearing the video lost it and Pixi warned about the
+ * destroyed source on the way back.
+ */
+const KEY = `video:a1b2c3d4e5f6/${'c7'.repeat(32)}.mp4#0~0~1~1`;
+
+function fakeVideo() {
+  return {
+    muted: false,
+    playsInline: false,
+    preload: '',
+    src: '',
+    currentTime: 0,
+    loop: false,
+    paused: true,
+    addEventListener: () => {},
+    requestVideoFrameCallback: () => 0,
+    play: () => Promise.resolve(),
+    pause: () => {},
+    removeAttribute: () => {},
+    load: () => {},
+  };
+}
+
+const page = { hidden: false, listeners: [] as (() => void)[] };
+
+function setHidden(hidden: boolean): void {
+  page.hidden = hidden;
+  for (const listener of page.listeners) listener();
+}
+
+async function freshLibrary() {
+  const { VideoLibrary } = await import('../../src/viewport/VideoLibrary');
+  return new VideoLibrary();
+}
+
+describe('VideoLibrary sweeping', () => {
+  beforeEach(() => {
+    vi.useFakeTimers({ toFake: ['setInterval', 'clearInterval', 'performance'] });
+    page.hidden = false;
+    page.listeners = [];
+    vi.stubGlobal('document', {
+      get hidden() {
+        return page.hidden;
+      },
+      addEventListener: (_type: string, listener: () => void) => page.listeners.push(listener),
+      createElement: fakeVideo,
+    });
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    vi.unstubAllGlobals();
+  });
+
+  it('lets a player go that nobody asked for in 2 s', async () => {
+    const videos = await freshLibrary();
+    videos.get(KEY);
+    vi.advanceTimersByTime(4100);
+    expect(videos.debugInfo()).toEqual([]);
+  });
+
+  it('keeps it while the page is hidden, however long', async () => {
+    const videos = await freshLibrary();
+    videos.get(KEY);
+    setHidden(true);
+    vi.advanceTimersByTime(60_000);
+    expect(videos.debugInfo()).toHaveLength(1);
+  });
+
+  it('does not count the time hidden as idle once the page is back', async () => {
+    const videos = await freshLibrary();
+    videos.get(KEY);
+    setHidden(true);
+    vi.advanceTimersByTime(60_000);
+    setHidden(false);
+    vi.advanceTimersByTime(1900);
+    expect(videos.debugInfo()).toHaveLength(1);
+    vi.advanceTimersByTime(2200);
+    expect(videos.debugInfo()).toEqual([]);
+  });
+});