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/RHI/IGraphicsEngine.cs

@@ -50,10 +50,10 @@ public interface IGraphicsEngine : IDisposable
     void ClearRenderers();
 
     /// <summary>
-    /// Creates a new command allocator for the specified command buffer type.
+    /// Creates a new command allocator for the specified command buffer space.
     /// </summary>
-    /// <param name="type">The type of command buffer for which to create the allocator. The default is CommandBufferType.Graphics.</param>
-    /// <returns>An <see cref="ICommandAllocator"/> instance configured for the specified command buffer type.</returns>
+    /// <param name="type">The space of command buffer for which to create the allocator. The default is CommandBufferType.Graphics.</param>
+    /// <returns>An <see cref="ICommandAllocator"/> instance configured for the specified command buffer space.</returns>
     ICommandAllocator CreateCommandAllocator(CommandBufferType type = CommandBufferType.Graphics);
 
     /// <summary>