feat: rebuild Vulkan renderer from scratch — pure P/Invoke triangle (Vulkan 1.3)

- Complete rewrite of Engine.Graphics.Vulkan with pure P/Invoke (no wrapper libs)
- Vulkan 1.3: dynamic rendering (vkCmdBeginRendering/vkCmdEndRendering),
  synchronization2 (vkQueueSubmit2, vkCmdPipelineBarrier2)
- Split types into VulkanHandles.cs, VulkanEnums.cs, VulkanStructs.cs
- Staging buffer → device-local vertex buffer pattern
- Correct swapchain semaphore indexing (per-image, not per-frame-in-flight)
- VK_EXT_debug_utils debug messenger with validation layer fallback
- Dynamic viewport/scissor (no pipeline recreation on resize)
- Simplified Program.cs to triangle-only rendering
- Removed old Silk.NET renderer, ImGui, PBR shaders, screenshot code
- Updated VULKAN_IMPLEMENTATION_PLAN.md with full architecture decisions

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

@@ -1,19 +0,0 @@
-#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);
-}