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>

src/AcDream.App/Rendering/CameraController.cs

@@ -1,47 +1,81 @@
 // src/AcDream.App/Rendering/CameraController.cs
+using AcDream.Core.Rendering;
+
 namespace AcDream.App.Rendering;
 
 public sealed class CameraController
 {
-    public OrbitCamera Orbit { get; }
-    public FlyCamera Fly { get; }
-    public ChaseCamera? Chase { get; private set; }
-    public ICamera Active { get; private set; }
-    public bool IsFlyMode => Active == Fly;
-    public bool IsChaseMode => Chase is not null && Active == Chase;
+    public OrbitCamera        Orbit       { get; }
+    public FlyCamera          Fly         { get; }
+    public ChaseCamera?       Chase       { get; private set; }
+    public RetailChaseCamera? RetailChase { get; private set; }
+
+    /// <summary>
+    /// The renderer-facing active camera. In chase mode, returns
+    /// <see cref="RetailChase"/> when
+    /// <see cref="CameraDiagnostics.UseRetailChaseCamera"/> is true,
+    /// otherwise <see cref="Chase"/>. In fly mode returns
+    /// <see cref="Fly"/>; default is <see cref="Orbit"/>.
+    /// </summary>
+    public ICamera Active
+    {
+        get
+        {
+            if (_mode == Mode.Fly) return Fly;
+            if (_mode == Mode.Chase)
+            {
+                if (CameraDiagnostics.UseRetailChaseCamera && RetailChase is not null)
+                    return RetailChase;
+                if (Chase is not null) return Chase;
+            }
+            return Orbit;
+        }
+    }
+
+    public bool IsFlyMode   => _mode == Mode.Fly;
+    public bool IsChaseMode => _mode == Mode.Chase;
 
     public event Action<bool>? ModeChanged;
 
+    private enum Mode { Orbit, Fly, Chase }
+    private Mode _mode = Mode.Orbit;
+
     public CameraController(OrbitCamera orbit, FlyCamera fly)
     {
         Orbit = orbit;
-        Fly = fly;
-        Active = orbit;
+        Fly   = fly;
     }
 
     public void ToggleFly()
     {
-        Active = IsFlyMode ? (ICamera)Orbit : Fly;
+        _mode = IsFlyMode ? Mode.Orbit : Mode.Fly;
         ModeChanged?.Invoke(IsFlyMode);
     }
 
-    public void EnterChaseMode(ChaseCamera chase)
+    /// <summary>
+    /// Enter chase mode with both candidate cameras. Both are held;
+    /// <see cref="Active"/> picks based on
+    /// <see cref="CameraDiagnostics.UseRetailChaseCamera"/>.
+    /// </summary>
+    public void EnterChaseMode(ChaseCamera legacy, RetailChaseCamera retail)
     {
-        Chase = chase;
-        Active = chase;
+        Chase       = legacy;
+        RetailChase = retail;
+        _mode       = Mode.Chase;
         ModeChanged?.Invoke(IsChaseMode);
     }
 
     public void ExitChaseMode()
     {
-        Active = Fly;
-        Chase = null;
+        Chase       = null;
+        RetailChase = null;
+        _mode       = Mode.Fly;
         ModeChanged?.Invoke(IsFlyMode);
     }
 
     public void SetAspect(float aspect)
     {
         Orbit.Aspect = aspect;
-        Fly.Aspect = aspect;
+        Fly.Aspect   = aspect;
     }
 }

src/AcDream.App/Rendering/GameWindow.cs

@@ -9856,7 +9856,7 @@ public sealed class GameWindow : IDisposable
             && _playerMode
             && _chaseCamera is not null)
         {
-            _cameraController.EnterChaseMode(_chaseCamera);
+            _cameraController.EnterChaseMode(_chaseCamera, new RetailChaseCamera { Aspect = _chaseCamera.Aspect });
             return;
         }
         _cameraController.ToggleFly();
@@ -9962,7 +9962,7 @@ public sealed class GameWindow : IDisposable
         // accumulated value from a previous session doesn't leak
         // into a future code path that re-enables mouse-yaw.
         _playerMouseDeltaX = 0f;
-        _cameraController?.EnterChaseMode(_chaseCamera);
+        _cameraController?.EnterChaseMode(_chaseCamera, new RetailChaseCamera { Aspect = _chaseCamera.Aspect });
         // K-fix1 (2026-04-26): latch the "we have entered chase at least
         // once" flag so the live-mode pre-login render gate stops
         // suppressing the scene. From here on, the orbit camera (if the

src/AcDream.App/Rendering/RetailChaseCamera.cs

@@ -93,7 +93,7 @@ public sealed class RetailChaseCamera : ICamera
         Vector3 heading = ComputeHeading(avgVel, playerYaw + YawOffset, CameraDiagnostics.AlignToSlope);
 
         // 3. Orthonormal heading-frame basis.
-        var (forward, right, up) = BuildBasis(heading);
+        var (forward, _, up) = BuildBasis(heading);
 
         // 4. Target pose.
         Vector3 pivotWorld = playerPosition + new Vector3(0f, 0f, PivotHeight);