feat(camera): CameraController carries both legacy + retail chase cams

EnterChaseMode now takes (ChaseCamera, RetailChaseCamera); Active
consults CameraDiagnostics.UseRetailChaseCamera to pick which to
expose. Flag flip at runtime swaps cameras instantly (both are kept
warm). GameWindow's two EnterChaseMode call sites get a temporary
stub RetailChaseCamera; Task 7 wires proper construction +
per-frame updates.

Also folds in two minor cleanups from the Task 3 code review:
- Update() discards the unused `right` axis from BuildBasis (no
  caller in the chase-cam math; viewer_offset.X is always 0)
- The three CameraDiagnostics-mutating integration tests now
  save and restore the static state in try/finally to avoid
  ordering-dependent contamination

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/Rendering/CameraControllerTests.cs

@@ -0,0 +1,67 @@
+using System.Numerics;
+using AcDream.App.Rendering;
+using AcDream.Core.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class CameraControllerTests
+{
+    private static (CameraController ctl, ChaseCamera legacy, RetailChaseCamera retail) MakeChaseFixture()
+    {
+        var orbit  = new OrbitCamera();
+        var fly    = new FlyCamera();
+        var ctl    = new CameraController(orbit, fly);
+        var legacy = new ChaseCamera();
+        var retail = new RetailChaseCamera();
+        ctl.EnterChaseMode(legacy, retail);
+        return (ctl, legacy, retail);
+    }
+
+    [Fact]
+    public void ChaseMode_WhenFlagOff_ActiveIsLegacy()
+    {
+        CameraDiagnostics.UseRetailChaseCamera = false;
+        var (ctl, legacy, _) = MakeChaseFixture();
+        Assert.Same(legacy, ctl.Active);
+        Assert.True(ctl.IsChaseMode);
+    }
+
+    [Fact]
+    public void ChaseMode_WhenFlagOn_ActiveIsRetail()
+    {
+        CameraDiagnostics.UseRetailChaseCamera = true;
+        var (ctl, _, retail) = MakeChaseFixture();
+        Assert.Same(retail, ctl.Active);
+        Assert.True(ctl.IsChaseMode);
+
+        // Reset.
+        CameraDiagnostics.UseRetailChaseCamera = false;
+    }
+
+    [Fact]
+    public void ChaseMode_FlagFlipped_ActiveSwaps()
+    {
+        CameraDiagnostics.UseRetailChaseCamera = false;
+        var (ctl, legacy, retail) = MakeChaseFixture();
+        Assert.Same(legacy, ctl.Active);
+
+        CameraDiagnostics.UseRetailChaseCamera = true;
+        Assert.Same(retail, ctl.Active);
+
+        CameraDiagnostics.UseRetailChaseCamera = false;
+        Assert.Same(legacy, ctl.Active);
+    }
+
+    [Fact]
+    public void ExitChaseMode_ClearsBothCameras()
+    {
+        CameraDiagnostics.UseRetailChaseCamera = false;
+        var (ctl, _, _) = MakeChaseFixture();
+        ctl.ExitChaseMode();
+
+        Assert.Null(ctl.Chase);
+        Assert.Null(ctl.RetailChase);
+        Assert.False(ctl.IsChaseMode);
+    }
+}