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/triangle.frag

@@ -0,0 +1,8 @@
+#version 450
+
+layout(location = 0) in vec3 fragColor;
+layout(location = 0) out vec4 outColor;
+
+void main() {
+    outColor = vec4(fragColor, 1.0);
+}