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.

GhostEngine.slnx

@@ -34,6 +34,7 @@
       <Deploy />
     </Project>
     <Project Path="Ghost.RenderGraph.Concept/Ghost.RenderGraph.Concept.csproj" />
+    <Project Path="Ghost.Shader.Concept/Ghost.Shader.Concept.csproj" Id="23f21a60-bf61-4bb9-acf1-332a31322ee9" />
     <Project Path="Ghost.Shader.Test/Ghost.Shader.Test.csproj" />
     <Project Path="Ghost.Test.Core/Ghost.Test.Core.csproj" />
   </Folder>