Replace precompile with onBeforeCompile injection for Three.js materials

[AndrewRayCode]

Summary

Replaces the old renderer.compile() + RawShaderMaterial approach with onBeforeCompile on real Three.js materials (MeshPhysicalMaterial, MeshPhongMaterial, MeshToonMaterial).

• threngine: New createMaterial() using onBeforeCompile with an InjectionMap system that targets specific #include <chunk> replacements. Strips Three.js built-in attribute/varying declarations from user code to avoid redefinitions. Forces USE_UV when user code references vUv so Three.js's UV pipeline populates it correctly.
• graph.ts: New ThreeNodeInfo/ThreeNodeFiller types on CompileGraphResult. Engine nodes are skipped in compileNode (no void main generated). Output node sections are filtered from compileSource for Three engine graphs so fragmentResult/vertexResult contain only user-authored functions.
• context.ts: Early return for Three engine nodes — no AST needed since they no longer generate shader code.
• threeExport.ts: New self-contained JS export generator for both pure ShaderMaterial graphs and MeshPhysical/Phong/ToonMaterial graphs with baked chunk replacements.

Test plan

• Load /editor/create/three — checkerboard renders on sphere with no WebGL errors
• Verify fragment fillers (map, normalMap, etc.) inject correctly via InjectionMap
• Verify vertex position filler replaces begin_vertex chunk
• Confirm threeExport.ts generates valid JS for both shader types

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