Lay a shader over a particle source's whole field (#170)

A shader on a source runs per particle and stops at each quad, so a
glow or blur across the whole cloud was not possible. A source now has
a second shader row that is laid over everything it draws at once.

- New rows Field shader, Field width and Field height (256) on a
  particle source. The size rows show only while a field shader is
  picked (new `shownWhenSet` on a row definition).
- The particles are drawn into a RenderTexture of the frame, and a
  sprite mesh wearing the field shader draws that picture. The frame
  is centred on the source and turns and grows with it.
- UV run 0 to 1 across the frame. Texture color, Texture alpha and
  Sample texture read the picture of the particles.
- The picture has as many device pixels as the frame covers on
  screen, at most 2048 per side. A frame off screen draws no pass.
- The field shader's Padding and Vertex output do nothing: the frame
  is the edge, and its corners stay put as on a nine-sliced sprite.
- A name both shaders declare is one row, and both uniforms read it.
- The scene view draws the frame dashed round a selected source. The
  Particles tab preview wears the field shader too.
- `tools/perf-field-shaders.mjs` measures 1, 10 and 50 sources. On
  the HD 630 they ran at 60.0, 60.0 and 57.6 fps against 60.1, 60.0
  and 59.2 without, one run per row.

Tests:
- Gate: 223 files, 3460 tests (3452 + 8), CHECK EXIT 0.
- e2e 31: 43 checks, 3 of them new. Sum 1707 is computed (1704 + 3).

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

src/editor/debug/renderHooks.ts

@@ -191,6 +191,9 @@ export function renderHooks({ app, state, editor }: HookContext) {
     /** What a source is drawing in the scene tab, or in the game view while playing. */
     particlesOf: (objectId: string, where: 'scene' | 'game' = 'scene', limit = 8) =>
       (where === 'game' ? gameView : viewport).particleInfo(objectId, limit),
+    /** The device pixels of a source's field-shader picture, null while none is drawn (#170). */
+    fieldPixels: (objectId: string, where: 'scene' | 'game' = 'scene') =>
+      (where === 'game' ? gameView : viewport).fieldPixels(objectId),
     /** Graphs compiled so far; a moved node must not add one (#65). */
     shaderCompiles: () => shaderLibrary.compiles,
     /** Which views redraw every tick because a Time shader is on screen. */