Refactor material palette system with GPU indirection

Major overhaul of material palette management:
- Added two-buffer indirection (PaletteOffsetBuffer, MaterialIndexBuffer) for GPU material lookup, with incremental upload and resizing.
- MaterialPaletteStore now tracks dirty ranges, supports deferred slot reclamation, and exposes CPU-side arrays for upload.
- ResourceManager manages persistent GPU buffers and uploads only dirty subranges per frame.
- Updated HLSL and C# structs to use palette indices.
- Refactored systems/components to use new palette index and release logic.
- Added RenderContext.UploadBufferRange for partial uploads.
Minor: Fixed StbIApi interop signatures, updated test namespaces, and performed code cleanups.

src/Runtime/Ghost.Graphics.RHI/RootSignatureLayout.cs

@@ -35,6 +35,8 @@ public struct FrameData
 {
     public uint instanceBuffer;
     public uint userBuffer;
+    public uint paletteOffsetBuffer;   // bindless index into PaletteOffsetBuffer
+    public uint materialIndexBuffer;   // bindless index into MaterialIndexBuffer
 }
 
 [StructLayout(LayoutKind.Sequential, Pack = 4)]
@@ -42,7 +44,7 @@ public struct InstanceData
 {
     public float4x4 localToWorld;
     public uint meshBuffer;
-    public uint materialBuffer;
+    public uint materialPaletteIndex;  // index into PaletteOffsetBuffer (from MaterialPaletteStore)
 }
 
 [StructLayout(LayoutKind.Sequential, Pack = 4)]
@@ -68,4 +70,5 @@ public struct MeshData
     public uint meshletBuffer;
     public uint meshletVerticesBuffer;
     public uint meshletTrianglesBuffer;
+    public uint materialSlotCount;     // number of material slots baked into this mesh's meshlets
 };