feat: adjustable ambient lighting via ImGui sliders

- SceneUBO expanded: added vec4 ambientColor at offset 400
- UBO size: 448 → 464 bytes
- Fragment shader: ambient from UBO instead of hardcoded constant
- VulkanRenderer: AmbientColor property (default 0.01, 0.01, 0.02)
- ImGui: R/G/B sliders for ambient (0.0-0.5 range)
- Packed at offset 400 after 4 shadowParams (64 bytes at 336)

src/CortexEngine.App/Program.cs

@@ -281,14 +281,21 @@ class Program
                     var bias = vkRenderer.ShadowBias;
                     var sampleRadius = vkRenderer.ShadowSampleRadius;
                     var farPlane = vkRenderer.ShadowFarPlane;
+                    var ambient = vkRenderer.AmbientColor;
 
                     ImGui.SliderFloat("Shadow Bias", ref bias, 0.0001f, 0.1f, "%.4f");
                     ImGui.SliderFloat("Sample Radius", ref sampleRadius, 0.001f, 0.1f, "%.4f");
                     ImGui.SliderFloat("Far Plane", ref farPlane, 10.0f, 120.0f, "%.1f");
+                    ImGui.Separator();
+                    ImGui.Text("Ambient Light");
+                    ImGui.SliderFloat("Ambient R", ref ambient.X, 0.0f, 0.5f, "%.3f");
+                    ImGui.SliderFloat("Ambient G", ref ambient.Y, 0.0f, 0.5f, "%.3f");
+                    ImGui.SliderFloat("Ambient B", ref ambient.Z, 0.0f, 0.5f, "%.3f");
 
                     vkRenderer.ShadowBias = bias;
                     vkRenderer.ShadowSampleRadius = sampleRadius;
                     vkRenderer.ShadowFarPlane = farPlane;
+                    vkRenderer.AmbientColor = ambient;
 
                     ImGui.Text($"Current: bias={bias:F4} radius={sampleRadius:F4} far={farPlane:F1}");
 

src/Engine.Graphics.Vulkan/Shaders/triangle.frag

@@ -18,6 +18,7 @@ layout(set = 0, binding = 0) uniform SceneUBO {
     vec2 padding;
     LightData lights[8];
     vec4 shadowParams[4];
+    vec4 ambientColor; // xyz = ambient color, w = unused
 } scene;
 
 layout(set = 0, binding = 1) uniform samplerCubeArray shadowArray;
@@ -151,7 +152,7 @@ void main()
     float roughness = 0.5;
     float metallic = 0.1;
 
-    vec3 color = AMBIENT * albedo;
+    vec3 color = scene.ambientColor.xyz * albedo;
 
     for (int i = 0; i < scene.numLights; i++)
     {

src/Engine.Graphics.Vulkan/VulkanFrameResources.cs

@@ -5,7 +5,7 @@ namespace Engine.Graphics.Vulkan;
 internal sealed unsafe class VulkanFrameResources : IDisposable
 {
     public const int MaxFramesInFlight = 2;
-    public const ulong UboSize = 448;
+    public const ulong UboSize = 464;
 
     public VkCommandPool CommandPool;
     public VkCommandBuffer[] CommandBuffers = new VkCommandBuffer[MaxFramesInFlight];

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -32,6 +32,7 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
     public float ShadowBias { get; set; } = 0.0001f;
     public float ShadowSampleRadius { get; set; } = 0.015f;
     public float ShadowFarPlane { get; set; } = 60.0f;
+    public Vector3 AmbientColor { get; set; } = new(0.01f, 0.01f, 0.02f);
 
     public bool IsRecording { get; set; }
     public byte[]? CapturedFrame { get; private set; }
@@ -227,6 +228,10 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
             System.Buffer.MemoryCopy(&sp, uboData + 336 + i * 16, 16, 16);
         }
 
+        // Ambient color at offset 400 (after 4 shadowParams = 64 bytes)
+        var ambient = new Vector4(AmbientColor, 0);
+        System.Buffer.MemoryCopy(&ambient, uboData + 400, 16, 16);
+
         // Compute light view-proj for each shadow light
         var lightViewProjs = new Matrix4x4[numShadowLights];
         for (int i = 0; i < numShadowLights; i++)