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/D3D12ResourceAllocator.cs

@@ -931,7 +931,10 @@ internal sealed unsafe partial class D3D12ResourceAllocator : IResourceAllocator
         ObjectDisposedException.ThrowIf(_disposed, this);
 
         var material = new Material();
-        material.SetShader(shader, this, _resourceDatabase);
+        if (material.SetShader(shader, this, _resourceDatabase) != ErrorStatus.None)
+        {
+            return Handle<Material>.Invalid;
+        }
 
         return _resourceDatabase.AddMaterial(in material);
     }