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/RenderGraphTypes.cs

@@ -0,0 +1,75 @@
+namespace Ghost.RenderGraph.Concept;
+
+/// <summary>
+/// Opaque handle to a render graph texture resource.
+/// </summary>
+public readonly struct RenderGraphTextureHandle : IEquatable<RenderGraphTextureHandle>
+{
+    public readonly int Index;
+    public readonly int Version;
+    internal readonly string InternalName;
+
+    public RenderGraphTextureHandle(int index, int version, string name = "")
+    {
+        Index = index;
+        Version = version;
+        InternalName = name;
+    }
+
+    public string Name => InternalName;
+
+    public bool IsValid() => Index >= 0;
+
+    public readonly bool Equals(RenderGraphTextureHandle other) => Index == other.Index && Version == other.Version;
+    public override readonly bool Equals(object? obj) => obj is RenderGraphTextureHandle other && Equals(other);
+    public override readonly int GetHashCode() => HashCode.Combine(Index, Version);
+    public static bool operator ==(RenderGraphTextureHandle left, RenderGraphTextureHandle right) => left.Equals(right);
+    public static bool operator !=(RenderGraphTextureHandle left, RenderGraphTextureHandle right) => !left.Equals(right);
+}
+
+/// <summary>
+/// Texture formats supported by the render graph.
+/// </summary>
+public enum TextureFormat : int
+{
+    RGBA8,
+    RGBA16F,
+    RGBA32F,
+    Depth32F,
+    Depth24Stencil8
+}
+
+/// <summary>
+/// Descriptor for creating a texture resource.
+/// </summary>
+public readonly struct TextureDescriptor : IEquatable<TextureDescriptor>
+{
+    public readonly int Width;
+    public readonly int Height;
+    public readonly TextureFormat Format;
+    public readonly string Name;
+
+    public TextureDescriptor(int width, int height, TextureFormat format, string name)
+    {
+        Width = width;
+        Height = height;
+        Format = format;
+        Name = name;
+    }
+
+    public readonly bool Equals(TextureDescriptor other) =>
+        Width == other.Width &&
+        Height == other.Height &&
+        Format == other.Format &&
+        Name == other.Name;
+
+    public override readonly bool Equals(object? obj) => obj is TextureDescriptor other && Equals(other);
+    public override readonly int GetHashCode() => HashCode.Combine(Width, Height, Format, Name);
+}
+
+/// <summary>
+/// Base interface for pass data that can be stored in the blackboard.
+/// </summary>
+public interface IPassData
+{
+}