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/debugHook.ts

@@ -417,6 +417,21 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
       center: () => centreOf(editor.shaderEditor.canvas),
       snapping: () => editor.shaderEditor.canvas.snapEnabled,
       snapButton: () => snapButton(editor.shaderEditor.snap),
+      /** The panel beside the graph (#168): what it shows, its clock and the sliders' values. */
+      preview: () => ({
+        ...editor.shaderEditor.controls,
+        holding: editor.shaderEditor.preview.holding,
+        seconds: Math.round(editor.shaderEditor.preview.time * 100) / 100,
+      }),
+      previewRect: () => rectOf(editor.shaderEditor.preview),
+      /** Where the picture sits inside the panel, in screen pixels. */
+      pictureRect: () => editor.shaderEditor.preview.pictureRect,
+      sliderAt: (key: string) => {
+        const slider = editor.shaderEditor.sliderOf(key);
+        return slider ? rectOf(slider) : null;
+      },
+      splitterAt: () => centreOf(editor.shaderEditor.splitter),
+      hold: (at: number | null) => editor.shaderEditor.hold(at),
     },
     /** The console panel: its lines as kept, its drawn height, and the divider above it. */
     console: {