feat: ImGui debug overlay — FPS, camera position, entity count

- ImGui.NET 1.91.6.1 integrated into Engine.Graphics.Vulkan
- VulkanImGui.cs: font atlas upload (staging → VkImage), sampler,
  descriptor set (COMBINED_IMAGE_SAMPLER), separate pipeline with
  alpha blending, no depth write, dynamic scissor
- ImGui shaders: imgui.vert (ortho MVP push constant) + imgui.frag
  (font texture sampler)
- Dynamic vertex/index buffers (HOST_VISIBLE, grow on demand)
- IRenderer interface: BeginImGuiFrame/EndImGuiFrame default methods
- Program.cs: debug window with FPS, camera pos/target, entity count
- 0 critical validation errors, ~750 FPS with physics + ImGui

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

@@ -0,0 +1,18 @@
+#version 450
+
+layout(location = 0) in vec2 inPosition;
+layout(location = 1) in vec2 inUV;
+layout(location = 2) in vec4 inColor;
+
+layout(location = 0) out vec4 fragColor;
+layout(location = 1) out vec2 fragUV;
+
+layout(push_constant) uniform PC {
+    mat4 mvp;
+} pc;
+
+void main() {
+    gl_Position = pc.mvp * vec4(inPosition, 0.0, 1.0);
+    fragColor = inColor;
+    fragUV = inUV;
+}