Play through the first camera that is switched on

The game view took whatever camera came first in the scene, ignoring the
switch, so `enabled = false` on a camera did nothing and a second camera
never got its turn. `World.activeCamera()` now decides: of the cameras
switched on – the whole parent chain counted, as for any object – the
first in hierarchy order, and none on renders nothing at all. It is asked
per frame, so a graph switching cameras hands the view over immediately.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -29,8 +29,10 @@ Live: <https://nockal.com/game-maker/> · Widget playground: <https://nockal.com
 **The camera defines what is rendered.** A camera asset has a view size (`width` × `height`,
 default 640×360) and a `zoom`; the frame it covers in world units is `size / zoom`. The Scene
 tab draws that frame as an outline. The Game tab renders only that frame – fitted into the
-view with black letterboxing – and objects outside it are culled. A scene without a camera
-renders nothing in the Game tab. Play has no built-in behaviour (no input, no movement):
+view with black letterboxing – and objects outside it are culled. Of several cameras the first
+one in hierarchy order that is **switched on** (`enabled`, its parents included) is the active
+one, so switching it off hands the view to the next; with none on nothing is rendered at all.
+Play has no built-in behaviour (no input, no movement):
 everything that happens is defined by behavior graphs.
 
 **Editing while playing.** Play opens the Game tab but leaves the editor fully usable: the
@@ -448,7 +450,7 @@ npm run format       # Prettier over src, tests and tools
 npm run check        # typecheck + lint + format:check + tests, what CI would run
 npm run build        # writes dist/ and regenerates index.html
 export GM_REMOTE=user@host                                    # where the browser runs; see below
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 330 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 337 editor checks, screenshots in /tmp/gm-e2e-*.png
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs   # 24 checks: the example game, played end to end
 ```