Add ShaderGen module analysis and incremental refactoring plan

[Copilot]

Analyzed the ShaderGen module (51 files, ~5,800 LOC) and created a structured refactoring plan addressing architectural debt while maintaining continuous buildability.

Analysis

Core Components Identified:

• ShaderCreateInfo hierarchy - per-stage shader generation (Vertex/Geometry/Fragment)
• MaterialCreateInfo - multi-stage coordinator
• MaterialDescriptorInfo / ShaderDescriptorInfo - resource management with auto set/binding allocation
• GLSLCompiler - SPIR-V compilation via dynamic plugin
• StdMaterial hierarchy - 2D/3D material framework

Key Strengths:

• Automated descriptor binding allocation
• Material instancing for efficient batching
• Dynamic shader composition from config

Critical Issues:

• Mixed responsibilities: ShaderCreateInfo handles both generation and compilation
• String-concatenation-based code generation scattered across classes
• No shader caching, no parallel compilation
• Tight coupling prevents testing in isolation
• Extension requires invasive changes (e.g., Task/Mesh shaders, HLSL support)

Refactoring Plan

25 incremental steps across 8 phases, each step maintains buildability:

Phase 1 (P1) - Foundation: Test harness, structured error reporting, config extraction
Phase 2 (P1) - Generation: ShaderCodeBuilder abstraction, eliminate direct string ops
Phase 3 (P1) - Descriptors: Clarify MDI/SDI split, fluent API, validation
Phase 4 (P1) - Decoupling: IShaderCompiler interface, builder pattern, dependency injection
Phase 5 (P2) - Extension: Template system, shader cache, material variants
Phase 6 (P2) - Performance: Object pooling, parallel compilation, profiling
Phase 7 (P3) - Tooling: API docs, shader visualizer, material editor
Phase 8 (P3) - Advanced: Mesh/Task shaders, HLSL support, SPIRV-Tools integration

Example transformation:

// Current: direct string concatenation
bool ShaderCreateInfo::ProcUBO() {
    final_shader += "\nlayout(set=...";
}

// Target: structured builder
bool ShaderCreateInfo::ProcUBO(ShaderCodeBuilder& builder) {
    for (auto& ubo : ubo_list)
        builder.addUBO(*ubo);
}

Timeline: 3-4 months single-engineer, 1.5-2 months small team

Document: doc/ShaderGen_Refactoring_Plan.md

Original prompt

分析其中的ShaderGen模块，评价其优缺点。并给出详尽可行的重构计划，可以将重构计划切分成十分细的流程，要求是每重构一步都是可以重新编译运行的。也就是不会出现超大规模的一次性重构。将重构计划生成.md文档放在工程中。

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