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.Shader/Generator/ShaderStructGenerator.cs

@@ -55,12 +55,12 @@ internal static partial class ShaderStructGenerator
         }
 
         var enumName = type.Name;
-        //var underlyingType = Enum.GetUnderlyingType(type);
+        //var underlyingType = Enum.GetUnderlyingType(space);
         //var underlyingTypeName = underlyingType switch
         //{
         //    Type t when t == typeof(byte) || t == typeof(short) || t == typeof(int) => "int",
         //    Type t when t == typeof(sbyte) || t == typeof(ushort) || t == typeof(uint) => "uint",
-        //    _ => throw new InvalidOperationException($"Unsupported underlying type {underlyingType.FullName} for enum {enumName}."),
+        //    _ => throw new InvalidOperationException($"Unsupported underlying space {underlyingType.FullName} for enum {enumName}."),
         //};
 
         //        sb.Append(@$"