Share a link that plays the game without the editor (#266)

A project could only be played inside the editor, behind the login.
To show a game to someone else there was no way short of an account.
A release freezes the saved project under a random token and serves
it on a page that holds nothing but the game view.

- Menu -> Release... opens a dialog with the state of the release,
  the link with Copy link and Open, Publish or Update release, and
  Take down behind a confirmation.
- Publish saves first and refuses while anything is unsaved, like
  snapshots. Updating keeps the link. Take down kills it, and a new
  Publish hands out a new token.
- The link is play.html?game=<32 hex>. The page needs no session,
  paints Theme.colors.background, zooms with Auto and waits for
  "Click to play" so that sound and keyboard focus are allowed.
- The game always starts in the start scene. Persistent variables
  are stored per link in the browser. Plugins are not loaded, and the
  dialog warns about them and about a project without a scene.
- GET ?r=releases/{token} is the only route besides auth/* that works
  without a login. The public answer carries the token as project.id,
  so the project id stays hidden. Release files are written with 0640
  and nginx does not serve them directly (measured 403).
- Starting, the frame and Load scene moved into runtime/GameRun.ts.
  Play in the editor and the play page run through the same code.
- WorldViewport moved to src/viewport/ and no longer needs the
  EditorState. Pointer forwarding (gamePointer.ts) and setting up
  textures, fonts and sounds (projectFiles.ts) exist only once.
- Vite builds two pages. The play page loads 4 kB of its own code
  plus the shared chunk (Pixi, runtime, drawing, 653 kB) and Rapier,
  without the editor.

Found on the way, not fixed here: data/projects/<id>.json answers 200
without a login because the files are 0644.

Tests:
- ./check.sh: 194 files, 2808 tests, exit 0.
- tests/php/releases.test.php: 20 checks, also added to CI.
- New e2e module 60 with 10 checks, played in a browser without a
  session. Module 46 counts the menu entries and was updated.
- Full e2e run: 1572 checks passed.
- Counter-checks: 3 unit, 2 PHP and 3 browser mutations all fail.

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

src/ui/index.ts

@@ -12,6 +12,7 @@ export {
   renderCount,
   requestRender,
   resetRenderLoop,
+  stagePixel,
 } from './render';
 export {
   drawBand,

src/ui/render.ts

@@ -1,4 +1,4 @@
-import { type Application, UPDATE_PRIORITY } from 'pixi.js';
+import { type Application, Rectangle, UPDATE_PRIORITY } from 'pixi.js';
 
 /**
  * Drives the renderer from changes instead of from the clock.
@@ -128,3 +128,10 @@ export function resetRenderLoop(): void {
   frameWaiters = [];
   frameWork = new Map();
 }
+
+/** The colour actually on screen at a point, as `#rrggbb` – what was drawn, not what the model says. */
+export function stagePixel(app: Application, x: number, y: number): string {
+  const { pixels } = app.renderer.extract.pixels({ target: app.stage, frame: new Rectangle(x, y, 1, 1) });
+  const hex = (value: number | undefined) => (value ?? 0).toString(16).padStart(2, '0');
+  return `#${hex(pixels[0])}${hex(pixels[1])}${hex(pixels[2])}`;
+}