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.Core/Graphics/ShaderDescriptor.cs

@@ -1,9 +1,9 @@
 namespace Ghost.Core.Graphics;
 
-public enum KeywordType
+public enum KeywordSpace
 {
-    Static,
-    Dynamic,
+    Local,
+    Global,
 }
 
 public enum ShaderPropertyType
@@ -29,7 +29,7 @@ public struct ShaderEntryPoint
 
 public struct KeywordsGroup
 {
-    public KeywordType type;
+    public KeywordSpace space;
     public List<string>? keywords;
 }