Show each camera only its layers and lay overlay cameras on top (#302)

A camera could only show the whole scene, so a HUD drawn in the world
would run through a camera shader (#299) with everything else. Render
layers say which cameras see an object, and overlay cameras draw their
own picture over the main one.

- The project has a list of up to 16 render layers, edited in Project
  settings next to the collision layers. It is a list of its own, read
  by the same rules (`layerNamesOf`).
- Sprites, texts, tilemaps, particle sources and containers carry a row
  `Render layer` under their switches. Its default `Inherit` takes the
  parent's layer, at the top the first one.
- A camera has `Sees` (all layers by default) and `Overlay`. The main
  camera is the first one switched on without Overlay, so two plain
  cameras still draw through the first one only.
- Every view through a camera keeps only what that camera sees and lays
  one see-through view per overlay camera over itself. That view is a
  `WorldViewport` of its own, handed its view and frame from below.
- An overlay is fitted into the main camera's whole picture without
  distortion. One of the same size and zoom covers it exactly.
- `z` sorts within one camera only. The pictures lie on top of each
  other as a whole, which is what a shader per camera (#299) needs.
- The Game tab's preview now collects the scene once per redraw. Before
  it collected it twice, with the cameras it would have been five times.
- `SetCollisionLayersCommand` became `SetLayerNamesCommand`, which
  takes the list it sets.

Follow-ups: #304 (the mouse through a chosen camera) and #305 (change
the render layer at run time).

Tests:
- Gate: 226 files, 3487 tests (3480 + 7)
- e2e 63 (new): 4 checks. A green overlay sprite at z -10 covers a
  red main sprite at z 1000, in the preview and in the running game.
  A `UI` sprite in front of the main camera and a `Default` one in
  front of the overlay stay away. Without Overlay the second camera
  draws nothing. The settings dialog adds a render layer at the end.
- Counter-checks: inheriting from parents switched off fails 1 unit
  test. Overlays never drawn fails the two overlay checks in e2e 63.
- e2e total 1720 (1716 + 4), computed.

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

src/core/store/serialize.ts

@@ -45,6 +45,7 @@ import type {
 import { parseCellKey } from '../model/tilemap';
 import { pluginsOf } from './projectPlugins';
 import { collisionLayersOf } from '../model/collisionLayers';
+import { renderLayersOf } from '../model/renderLayers';
 import { gameLinkZoomOf, gameScaleOf, playFromOf, whilePlayingOf } from '../model/playSettings';
 import { sceneVisibilityOf } from '../model/sceneVisibility';
 import { symbolScaleOf } from '../model/symbolScale';
@@ -284,6 +285,7 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
     // is actually running.
     plugins: pluginsOf(r),
     collisionLayers: collisionLayersOf(r),
+    renderLayers: renderLayersOf(r),
     symbolScale: symbolScaleOf(r),
     playFrom: playFromOf(r),
     whilePlaying: whilePlayingOf(r),