feat(render): Phase A8.F — add CameraDiagnostics.CollideCamera flag (default on)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/AcDream.Core/Rendering/CameraDiagnostics.cs

@@ -37,6 +37,17 @@ public static class CameraDiagnostics
     public static bool AlignToSlope { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_CAMERA_ALIGN_SLOPE") != "0";
 
+    /// <summary>
+    /// When true (default), the chase camera sweeps a 0.3 m collision
+    /// sphere from the head-pivot to the desired eye and stops it at the
+    /// first wall (retail <c>SmartBox::update_viewer</c> spring arm), so
+    /// the eye never sits behind/inside geometry. Initial state from
+    /// <c>ACDREAM_CAMERA_COLLIDE</c>; default-on if unset, off only when
+    /// explicitly set to <c>"0"</c>.
+    /// </summary>
+    public static bool CollideCamera { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_CAMERA_COLLIDE") != "0";
+
     /// <summary>
     /// Per-frame translation damping rate. Retail default 0.45. Higher
     /// (→ 1.0) snaps faster; lower (→ 0.0) lags more. Formula per frame:

tests/AcDream.Core.Tests/Rendering/CameraDiagnosticsTests.cs

@@ -46,4 +46,16 @@ public class CameraDiagnosticsTests
         CameraDiagnostics.TranslationStiffness = 0.45f;
         CameraDiagnostics.UseRetailChaseCamera = true;
     }
+
+    [Fact]
+    public void CollideCamera_DefaultOn_AndPersistsRuntimeChanges()
+    {
+        CameraDiagnostics.CollideCamera = true;
+        Assert.True(CameraDiagnostics.CollideCamera);
+
+        CameraDiagnostics.CollideCamera = false;
+        Assert.False(CameraDiagnostics.CollideCamera);
+
+        CameraDiagnostics.CollideCamera = true; // reset so other tests aren't poisoned
+    }
 }