添加 NanoVDB 体积渲染着色器

1. 复制 PNanoVDB.hlsl 到 Res/Shader/
   - GPU 端读取 NanoVDB 数据的核心库

2. 创建 volume.hlsl 体积渲染着色器
   - 包含 VS 和 PS 入口
   - 读取 NanoVDB 边界框数据
   - 已通过 fxc 编译验证

Res/Shader/volume.hlsl

@@ -0,0 +1,50 @@
+#define PNANOVDB_HLSL
+#define PNANOVDB_ADDRESS_64
+#include "PNanoVDB.hlsl"
+
+cbuffer CB0 : register(b1)
+{
+    float4x4 _ViewProjection;
+    float3 _CameraPos;
+    float _DensityScale;
+};
+
+StructuredBuffer<uint> buf : register(t1);
+
+struct VSInput
+{
+    float3 position : POSITION;
+};
+
+struct PSInput
+{
+    float4 position : SV_POSITION;
+    float3 worldPos : TEXCOORD0;
+};
+
+PSInput MainVS(VSInput input)
+{
+    PSInput output;
+    output.position = mul(_ViewProjection, float4(input.position, 1.0));
+    output.worldPos = input.position;
+    return output;
+}
+
+float4 MainPS(PSInput input) : SV_TARGET
+{
+    pnanovdb_grid_handle_t grid;
+    grid.address.byte_offset = 0;
+    
+    float3 worldBboxMin = float3(
+        (float)pnanovdb_grid_get_world_bbox(buf, grid, 0),
+        (float)pnanovdb_grid_get_world_bbox(buf, grid, 1),
+        (float)pnanovdb_grid_get_world_bbox(buf, grid, 2)
+    );
+    float3 worldBboxMax = float3(
+        (float)pnanovdb_grid_get_world_bbox(buf, grid, 3),
+        (float)pnanovdb_grid_get_world_bbox(buf, grid, 4),
+        (float)pnanovdb_grid_get_world_bbox(buf, grid, 5)
+    );
+    
+    return float4(0.0, 1.0, 0.0, 1.0);
+}