Add high-performance material/shader system (Ghost.Shader.Concept)

Introduces a new Ghost.Shader.Concept project implementing a modern, data-oriented material and shader system with:
- Global/local keyword bitsets (fast O(1) ops, 64 bytes)
- Multi-pass shader program and per-pass render state overrides
- Thread-safe, 16-byte aligned material property blocks
- Material pooling to reduce GC pressure
- Batch renderer for efficient PSO grouping and async variant warmup
- Full demo (Program.cs) and extensive documentation (ARCHITECTURE.md, README.md, PROJECT_SUMMARY.md)
- Minor integration: new enums, doc updates, and keyword handling in existing code

No breaking changes to the existing engine; all new code is isolated. This serves as a reference implementation for high-performance, extensible material/shader architectures.

Ghost.Graphics/Contracts/IShaderCompiler.cs

@@ -145,5 +145,5 @@ public interface IShaderCompiler : IDisposable
 {
     Result<ShaderCompileResult> Compile(ref readonly CompilerConfig config, Allocator allocator);
     Result<GraphicsCompiledResult> CompilePass(IPassDescriptor descriptor, string? generatedCodePath);
-    Result<GraphicsCompiledResult> LoadCompiledCache(ShaderPassKey key);
+    Result<GraphicsCompiledResult, ErrorStatus> LoadCompiledCache(ShaderPassKey key);
 }