Show only the game view in fullscreen, in the editor and the link (#267)

A game is easier to judge and to play without the editor around it.
The Game tab and the game link now both have a button that puts the
game view itself into fullscreen, not just the browser window.

- The Game tab header has a fullscreen button on the right. It hides
  toolbar, panels, tab headers and console and keeps the zoom from the
  picker. A running game keeps running.
- A corner button at the top right, or Esc, leaves fullscreen and puts
  the view back into the tab where it was.
- The game link shows the same corner button. Over a running game it
  hides after 2 s without mouse movement and returns on the next move.
- A click on the corner button never reaches the game.
- Stop ends fullscreen. Stop switches back to the Scene tab, and any
  switch away from the Game tab ends fullscreen, so no empty screen is
  left behind.
- src/viewport/fullscreen.ts wraps the Fullscreen API and holds the
  rule for when the button shows. FullscreenCorner.ts is the button
  used by both pages. Two SVG icons are new.
- Editor.showGameFullscreen moves the GameViewport into the root while
  fullscreen is on.

Tests:
- ./check.sh: 195 files, 2815 tests, exit 0 (3 new unit tests).
- e2e 60: 2 new checks for the corner button on the game link.
- e2e 61 (new): 4 checks for fullscreen in the editor.
- Full e2e run (local): ALL PASSED, 1580 checks.
- Counter-checks fail as expected: rule without "no game running",
  no stopPropagation (60h), no end on tab switch (61c), no reparenting
  of the view (61a).

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

src/editor/debug/hookHelpers.ts

@@ -1,4 +1,5 @@
-import type { Application } from 'pixi.js';
+import type { Application, Container } from 'pixi.js';
+import { ui } from '../../ui';
 import type { Editor } from '../Editor';
 import type { SnapToggle } from '../graph/SnapToggle';
 import type { EditorState } from '../state';
@@ -18,6 +19,16 @@ export function centreOf(widget: { getGlobalPosition(): { x: number; y: number }
   return { x: p.x + widget.w / 2, y: p.y + widget.h / 2 };
 }
 
+/** Whether `node` and everything it hangs from are visible, up to a stage. */
+export function shownOnStage(node: Container): boolean {
+  let current = node;
+  while (current.visible) {
+    if (!current.parent) return current === ui().app.stage;
+    current = current.parent;
+  }
+  return false;
+}
+
 /** A tab's magnet (#194): where to click it, how big it is drawn, and whether it is on. */
 export function snapButton(snap: SnapToggle): {
   x: number;