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/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