feat: ImGui integration + real screenshot capture via Vulkan

ImGui:
- ImGui.NET 1.91.6.1 NuGet package
- VulkanImGui: font texture upload, ImGui pipeline (blending, no depth), vertex/index buffer streaming
- GLSL 450 ImGui shaders (vert: scale/translate push constants, frag: font texture sampler)
- Compiled to SPIR-V via @webgpu/glslang
- Debug overlay: FPS, camera position, entity count, light list
- 1600+ FPS with ImGui active

Screenshot:
- vkCmdCopyImageToBuffer embedded in main command buffer
- Layout transition PresentSrcKHR→TransferSrcOptimal→PresentSrcKHR
- Deferred read: staging buffer read on next frame after fence
- BMP format written directly to FileStream (row by row)
- All 16 camera tour screenshots captured (1280x720x32bpp)

All 66 tests pass.

src/Engine.Graphics.Vulkan/Shaders/imgui.vert

@@ -0,0 +1,19 @@
+#version 450
+
+layout(location = 0) in vec2 inPos;
+layout(location = 1) in vec2 inUV;
+layout(location = 2) in vec4 inColor;
+
+layout(push_constant) uniform PushConstants {
+    vec2 scale;
+    vec2 translate;
+} pc;
+
+layout(location = 0) out vec2 fragUV;
+layout(location = 1) out vec4 fragColor;
+
+void main() {
+    fragUV = inUV;
+    fragColor = inColor;
+    gl_Position = vec4(inPos * pc.scale + pc.translate, 0.0, 1.0);
+}