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/graph.test.ts

@@ -74,6 +74,8 @@ describe('node registry', () => {
       expect(new Set(def.outputs.map((p) => p.key)).size).toBe(def.outputs.length);
       // An event is seeded by the runtime, a state is a name in a machine: neither runs.
       if (def.category === 'event' || def.category === 'state') expect(def.inputs).toHaveLength(0);
+      // A shader node is compiled to GLSL, never run.
+      else if (def.category === 'shader') expect('emit' in def).toBe(true);
       else expect(def.run).toBeTypeOf('function');
       expect(keys.every((k) => k.length > 0)).toBe(true);
     }