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/ShaderProgram.cs

@@ -0,0 +1,122 @@
+namespace Ghost.Shader.Concept;
+
+/// <summary>
+/// Represents a single rendering pass within a shader program.
+/// Each pass can have its own render state overrides.
+/// </summary>
+public sealed class ShaderPass
+{
+    public string Name { get; }
+    public int PassId { get; }
+    public RenderState RenderState { get; }
+    public string VertexEntryPoint { get; }
+    public string PixelEntryPoint { get; }
+
+    public ShaderPass(
+        string name,
+        int passId,
+        RenderState renderState,
+        string vertexEntryPoint = "VSMain",
+        string pixelEntryPoint = "PSMain")
+    {
+        Name = name;
+        PassId = passId;
+        RenderState = renderState;
+        VertexEntryPoint = vertexEntryPoint;
+        PixelEntryPoint = pixelEntryPoint;
+    }
+}
+
+/// <summary>
+/// Shader program containing multiple passes and keyword declarations.
+/// Immutable after creation for thread-safety.
+/// </summary>
+public sealed class ShaderProgram
+{
+    private static int _nextId = 0;
+
+    public int Id { get; }
+    public string Name { get; }
+    public ShaderPass[] Passes { get; }
+    public ShaderKeyword[] DeclaredKeywords { get; }
+    
+    private readonly Dictionary<string, int> _passNameToIndex = new();
+
+    public ShaderProgram(
+        string name,
+        ShaderPass[] passes,
+        ShaderKeyword[] declaredKeywords)
+    {
+        Id = Interlocked.Increment(ref _nextId);
+        Name = name;
+        Passes = passes;
+        DeclaredKeywords = declaredKeywords;
+
+        for (int i = 0; i < passes.Length; i++)
+        {
+            _passNameToIndex[passes[i].Name] = i;
+        }
+    }
+
+    public int GetPassIndex(string passName)
+    {
+        return _passNameToIndex.TryGetValue(passName, out int index) ? index : -1;
+    }
+
+    public ShaderVariantKey CreateVariantKey(in KeywordSet keywords)
+    {
+        return new ShaderVariantKey(Id, keywords.ComputeHash());
+    }
+}
+
+/// <summary>
+/// Builder pattern for creating shader programs fluently.
+/// </summary>
+public sealed class ShaderProgramBuilder
+{
+    private string _name = "Unnamed";
+    private readonly List<ShaderPass> _passes = new();
+    private readonly List<ShaderKeyword> _keywords = new();
+
+    public ShaderProgramBuilder WithName(string name)
+    {
+        _name = name;
+        return this;
+    }
+
+    public ShaderProgramBuilder AddPass(
+        string passName,
+        RenderState? renderState = null,
+        string vertexEntry = "VSMain",
+        string pixelEntry = "PSMain")
+    {
+        var pass = new ShaderPass(
+            passName,
+            _passes.Count,
+            renderState ?? RenderState.Default,
+            vertexEntry,
+            pixelEntry);
+        _passes.Add(pass);
+        return this;
+    }
+
+    public ShaderProgramBuilder DeclareKeyword(ShaderKeyword keyword)
+    {
+        _keywords.Add(keyword);
+        return this;
+    }
+
+    public ShaderProgramBuilder DeclareKeywords(params ShaderKeyword[] keywords)
+    {
+        _keywords.AddRange(keywords);
+        return this;
+    }
+
+    public ShaderProgram Build()
+    {
+        if (_passes.Count == 0)
+            throw new InvalidOperationException("Shader program must have at least one pass");
+
+        return new ShaderProgram(_name, _passes.ToArray(), _keywords.ToArray());
+    }
+}