GhostEngine Render Graph: major refactor & Unity RG ref

- Major architectural refactor for performance, extensibility, and feature completeness: resource pooling, pass culling, aliasing, and compilation caching.
- Introduces type-safe builder and context APIs, blackboard pattern, and unified resource management.
- Adds detailed documentation and cleans up obsolete files and APIs.
- Includes (commented) Unity Render Graph source for reference; not compiled, for parity and future extension.

Ghost.RenderGraph.Concept/RenderGraphBlackboard.cs

@@ -1,26 +1,43 @@
 namespace Ghost.RenderGraph.Concept;
 
-public class RenderGraphBlackboard
+/// <summary>
+/// Blackboard for sharing data between render passes.
+/// Uses a dictionary with type keys to store different pass data types.
+/// Avoids allocations by reusing the same dictionary across frames.
+/// </summary>
+public sealed class RenderGraphBlackboard
 {
-    private readonly Dictionary<Type, object> _data = new();
+    private readonly Dictionary<Type, object> _data = new(16);
 
-    public void Add<T>(T data) where T : class
+    /// <summary>
+    /// Adds or updates pass data in the blackboard.
+    /// </summary>
+    public void Add<T>(T data) where T : class, IPassData
     {
-        _data[typeof(T)] = data;
+        var type = typeof(T);
+        _data[type] = data;
     }
 
-    public T Get<T>() where T : class
+    /// <summary>
+    /// Retrieves pass data from the blackboard.
+    /// </summary>
+    public T Get<T>() where T : class, IPassData
     {
-        if (_data.TryGetValue(typeof(T), out var data))
+        var type = typeof(T);
+        if (_data.TryGetValue(type, out var obj))
         {
-            return (T)data;
+            return (T)obj;
         }
-        throw new KeyNotFoundException($"Data of type {typeof(T).Name} not found in blackboard.");
+        throw new KeyNotFoundException($"Pass data of type {type.Name} not found in blackboard");
     }
 
-    public bool TryGet<T>(out T? data) where T : class
+    /// <summary>
+    /// Tries to get pass data from the blackboard.
+    /// </summary>
+    public bool TryGet<T>(out T? data) where T : class, IPassData
     {
-        if (_data.TryGetValue(typeof(T), out var obj))
+        var type = typeof(T);
+        if (_data.TryGetValue(type, out var obj))
         {
             data = (T)obj;
             return true;
@@ -29,6 +46,10 @@ public class RenderGraphBlackboard
         return false;
     }
 
+    /// <summary>
+    /// Clears all data from the blackboard.
+    /// Does not deallocate the backing dictionary to avoid allocations.
+    /// </summary>
     public void Clear()
     {
         _data.Clear();