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.Concept/ShaderKeys.cs

@@ -0,0 +1,71 @@
+using System.Runtime.InteropServices;
+
+namespace Ghost.Shader.Concept;
+
+/// <summary>
+/// Unique identifier for a shader variant based on keyword combination.
+/// Used as key for shader compilation cache.
+/// </summary>
+[StructLayout(LayoutKind.Sequential)]
+public readonly struct ShaderVariantKey : IEquatable<ShaderVariantKey>
+{
+    public readonly int ShaderProgramId;
+    public readonly ulong KeywordHash;
+
+    public ShaderVariantKey(int shaderProgramId, ulong keywordHash)
+    {
+        ShaderProgramId = shaderProgramId;
+        KeywordHash = keywordHash;
+    }
+
+    public bool Equals(ShaderVariantKey other) =>
+        ShaderProgramId == other.ShaderProgramId && KeywordHash == other.KeywordHash;
+
+    public override bool Equals(object? obj) => obj is ShaderVariantKey other && Equals(other);
+    public override int GetHashCode() => HashCode.Combine(ShaderProgramId, KeywordHash);
+}
+
+/// <summary>
+/// Unique identifier for a graphics pipeline state object.
+/// Combines shader variant, render state, and pass information.
+/// </summary>
+[StructLayout(LayoutKind.Sequential)]
+public readonly struct GraphicsPipelineKey : IEquatable<GraphicsPipelineKey>
+{
+    public readonly ShaderVariantKey VariantKey;
+    public readonly ulong RenderStateHash;
+    public readonly int PassId;
+
+    public GraphicsPipelineKey(ShaderVariantKey variantKey, ulong renderStateHash, int passId)
+    {
+        VariantKey = variantKey;
+        RenderStateHash = renderStateHash;
+        PassId = passId;
+    }
+
+    public bool Equals(GraphicsPipelineKey other) =>
+        VariantKey.Equals(other.VariantKey) &&
+        RenderStateHash == other.RenderStateHash &&
+        PassId == other.PassId;
+
+    public override bool Equals(object? obj) => obj is GraphicsPipelineKey other && Equals(other);
+    public override int GetHashCode() => HashCode.Combine(VariantKey, RenderStateHash, PassId);
+}
+
+/// <summary>
+/// Mock interface for shader compiler (assumed to exist)
+/// </summary>
+public interface IShaderCompiler
+{
+    /// <summary>Compiles a shader variant for the given keyword set</summary>
+    IntPtr CompileVariant(ShaderVariantKey key, in KeywordSet keywords);
+}
+
+/// <summary>
+/// Mock interface for pipeline library (assumed to exist)
+/// </summary>
+public interface IPipelineLibrary
+{
+    /// <summary>Gets or creates a PSO for the given pipeline key</summary>
+    IntPtr GetOrCreatePipeline(in GraphicsPipelineKey key);
+}