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/PipelineState.cs

@@ -1,6 +1,6 @@
 namespace Ghost.Core.Graphics;
 
-public enum ZTest
+public enum ZTest : byte
 {
     Disabled,
     Less,
@@ -12,20 +12,20 @@ public enum ZTest
     Always
 }
 
-public enum ZWrite
+public enum ZWrite : byte
 {
     Off,
     On
 }
 
-public enum Cull
+public enum Cull : byte
 {
     Off,
     Front,
     Back
 }
 
-public enum Blend
+public enum Blend : byte
 {
     Opaque,
     Alpha,
@@ -35,7 +35,7 @@ public enum Blend
 }
 
 [Flags]
-public enum ColorWriteMask
+public enum ColorWriteMask : byte
 {
     None = 0,
     Red = 1 << 0,