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/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);