Shader assets: graph-built shaders, the Shader tab and sprite rows from the graph (#62)

A shader is an asset carrying a node graph (Texture color, Output, Property,
Color, Add/Subtract/Multiply/Divide, Mix, Luminance) that compiles to the GLSL
fragment of a Pixi filter. The Shader tab reuses the Behavior canvas; a sprite
wears a shader through its Shader picker, and the rows below it are the
graph's Property nodes. Property schema, resolution and deviation take the
project because a sprite's rows now live in another asset. The built-in
registry of the prototype is gone; "Adjust colors" ships as a graph in the
fixture and the three example games.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tools/seed-e2e.mjs

@@ -8,6 +8,7 @@
  */
 import { edge, node, port, sceneObject, variable } from './lib/builders.mjs';
 import { PROJECTS, cliArgs, writeProject } from './lib/projects.mjs';
+import { adjustColorsShader } from './lib/shaders.mjs';
 
 const { projectId, baseUrl } = cliArgs('seed-e2e.mjs', PROJECTS.fixture);
 
@@ -95,6 +96,8 @@ const project = {
       graph: moverGraph,
     },
     { id: 'spawner', type: 'behavior', name: 'Spawner', props: {}, variables: [], graph: spawnerGraph },
+    // Worn by nothing here; the shader step assigns it and reads the pixels.
+    adjustColorsShader(),
   ],
 };