The shader tab previews the effect on the last picked image (#168)

A shader could only be judged by placing it on a sprite and running
the scene. The shader tab now shows its effect next to the graph, on
the image the user picked last, without a picker to maintain.

- The preview follows the last selection that has an image: a placed
  sprite with its overrides, size and tint, a sprite asset, a texture
  or a frame. Clicking the shader, a node, a behavior or a scene keeps
  the picture.
- Without such a selection, or when the picked image is deleted, a
  built-in 64 px test image is shown. Its colour runs with u and v, a
  checkerboard shows where a UV lands and the border fades for alpha.
- Each property gets a slider. The sliders belong to the preview and
  never write to the project. They start at the sprite's values when
  it carries this shader, otherwise at the default.
- The preview has its own clock. Hold and Restart are only enabled
  when a Time node is in the graph.
- The graph | splitter | panel layout of the particles tab moves into
  PreviewSplit, and both tabs use it.
- A nine-sliced frame is previewed unsliced. This is documented.

Findings:

- The slider row stayed squeezed to a few pixels when the properties
  changed while the tab was hidden. The layout was triggered one level
  too high. The slider check in block 28n holds this.
- The graph canvas is narrower now, 385 px at a 1280 px window, the
  same as in the particles tab. Block 28b clicked at a fixed x+420 and
  now uses the measured width.

Tests:

- ./check.sh: 132 files, 1952 tests (was 131 / 1941), exit 0
- npm run build: exit 0
- GM_STEPS=28,31 freshly seeded: ALL PASSED, 105 checks
- 11 new unit tests in tests/editor/shader-preview.test.ts
- 7 new e2e checks in block 28n
- Counter-probes: start values ignoring the sprite fail 1 unit test,
  a preview ignoring the selection fails 2 e2e checks
- Check total 1209 in README is computed (1202 + 7), CI measures it

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

src/editor/Editor.ts

@@ -714,12 +714,13 @@ export class Editor implements PlayHost {
     // to do it, and its states move just the same.
     if (this.tab === 'statemachine' && this.editorRun.isRunning) this.stateMachineEditor.canvas.redraw();
     if (this.tab === 'particles') this.particlesEditor.tick();
+    if (this.tab === 'shader') this.shaderEditor.tick();
     if (this.movesByItself()) requestRender();
   }
 
   /**
    * What changes without anyone touching it, and therefore cannot be woken by an event: the
-   * running world, a Time shader on screen, the two previews that are moving pictures.
+   * running world, a Time shader on screen, the previews that are moving pictures.
    */
   private movesByItself(): boolean {
     return (
@@ -728,6 +729,7 @@ export class Editor implements PlayHost {
       (this.viewport.visible && this.viewport.animates) ||
       (this.gameView.visible && this.gameView.animates) ||
       this.tab === 'particles' ||
+      (this.tab === 'shader' && this.shaderEditor.animates) ||
       (this.tab === 'animation' && this.animationEditor.isPlaying) ||
       (this.tab === 'tile' && this.tileEditor.isPlaying)
     );