Add shader graph node system (sky::sg) for UE-style material blueprints

[Copilot]

Introduces a visual shader graph node system (sky::sg namespace) following UE material blueprint conventions, with HLSL code generation and Qt-based editor integration.

Core Data Model (engine/render/shader/include/shader/shadergraph/)

• ShaderGraphTypes.h — SGDataType (Float/Vec2/3/4/Texture2D/Sampler), MaterialSlot enum, HLSL type helpers
• ShaderGraphPin.h — SGPin, SGPinID, SGConnection for typed graph connectivity
• ShaderGraphNode.h — SGNode base class with pin management and SGCodeGenContext for HLSL emission
• Math nodes (14): Add, Subtract, Multiply, Divide, Lerp, Clamp, Saturate, Abs, Power, Dot, Cross, Normalize, ComponentMask, Append
• Input nodes: TexCoord, VertexColor, WorldPosition, WorldNormal, Time, Constants (Float/Vec2/3/4), Parameters (Scalar/Vector/Texture), TextureSample
• SGMaterialOutputNode — terminal node with PBR slots: BaseColor, Metallic, Roughness, Normal, Emissive, Opacity, OpacityMask
• ShaderGraph — graph container with connection management, topological HLSL codegen, JSON serialization, and a static node-type factory registry

// Build a graph programmatically
sg::ShaderGraph graph;
auto texParam = std::make_shared<sg::SGTextureParamNode>("Albedo");
auto uvNode   = std::make_shared<sg::SGTexCoordNode>();
auto sample   = std::make_shared<sg::SGTextureSampleNode>();
auto output   = std::make_shared<sg::SGMaterialOutputNode>();
graph.AddNode(texParam); graph.AddNode(uvNode);
graph.AddNode(sample);   graph.AddNode(output);
graph.AddConnection({{texParam->GetId(), 0}, {sample->GetId(), 0}}); // Texture → sample
graph.AddConnection({{uvNode->GetId(),   0}, {sample->GetId(), 2}}); // UV → sample
graph.AddConnection({{sample->GetId(),   1}, {output->GetId(), 0}}); // RGB → BaseColor

auto code = graph.GenerateHLSL();
// code.declarations: "Texture2D Albedo;\nSamplerState AlbedoSampler;\n"
// code.body:         per-pixel HLSL assignments into `surface.*`

Asset Integration (engine/render/adaptor/)

• ShaderGraphAsset.h/cpp — ShaderGraphAssetData with AssetTraits<ShaderGraphAssetTag> (type "ShaderGraph", .shadergraph extension)

Editor UI (engine/render/editor/)

• ShaderGraphNodeModel — QtNodes NodeDelegateModel wrappers; SGConcreteNodeModel<T> template for zero-boilerplate default-constructible models; embedded widgets for parameter naming and constant value editing
• ShaderGraphWidget — DataFlowGraphModel-backed visual editor; nodes organized by category (Math / Input / Constant / Parameter / Texture / Output)
• ShaderGraphEditWindow — IAssetPreviewWndFactory for asset browser integration
• ShaderGraphCreator — AssetCreatorBase registered as "Shader Graph" in RenderEditorModule

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