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>

tests/core/texture-commands.test.ts

@@ -54,6 +54,7 @@ describe('CreateAssetCommand with starting props', () => {
       height: 32,
       color: 0xffffff,
       texture: texture.asset.id,
+      shader: '',
     });
     stack.undo();
     expect(findAsset(project, sprite.asset.id)).toBeUndefined();