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/D3D12/D3D12ResourceDatabase.cs

@@ -464,7 +464,7 @@ internal class D3D12ResourceDatabase : IResourceDatabase
             ThrowMemoryLeakException("materials", _materials.Count);
         }
 
-        // SDL are reference type, it will be managed by GC, so we don't throw exception here.
+        // SDL are reference space, it will be managed by GC, so we don't throw exception here.
         for (var i = 0; i < _shaders.Count; i++)
         {
             ref var shader = ref _shaders[i];