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/GlobalKeywordState.cs

@@ -0,0 +1,71 @@
+namespace Ghost.Shader.Concept;
+
+/// <summary>
+/// Global keyword state manager. Singleton pattern for engine-wide keywords.
+/// Keywords like platform settings, quality levels, etc.
+/// </summary>
+public sealed class GlobalKeywordState
+{
+    private static readonly Lazy<GlobalKeywordState> _instance = new(() => new GlobalKeywordState());
+    public static GlobalKeywordState Instance => _instance.Value;
+
+    private KeywordSet _keywords;
+    private readonly object _lock = new();
+    private int _version = 0;
+
+    public int Version => _version;
+
+    private GlobalKeywordState()
+    {
+        _keywords = new KeywordSet();
+    }
+
+    public void EnableKeyword(ShaderKeyword keyword)
+    {
+        if (keyword.Scope != KeywordScope.Global)
+            throw new ArgumentException("Only global keywords can be set", nameof(keyword));
+
+        lock (_lock)
+        {
+            _keywords.Enable(keyword);
+            _version++;
+        }
+    }
+
+    public void DisableKeyword(ShaderKeyword keyword)
+    {
+        if (keyword.Scope != KeywordScope.Global)
+            throw new ArgumentException("Only global keywords can be set", nameof(keyword));
+
+        lock (_lock)
+        {
+            _keywords.Disable(keyword);
+            _version++;
+        }
+    }
+
+    public bool IsKeywordEnabled(ShaderKeyword keyword)
+    {
+        lock (_lock)
+        {
+            return _keywords.IsEnabled(keyword);
+        }
+    }
+
+    public KeywordSet GetKeywordSet()
+    {
+        lock (_lock)
+        {
+            return _keywords; // struct copy
+        }
+    }
+
+    public void Clear()
+    {
+        lock (_lock)
+        {
+            _keywords.Clear();
+            _version++;
+        }
+    }
+}