feat(camera): InputAction + DebugVM surface for retail chase camera

Four new InputAction entries for held-key offset integration
(CameraZoomIn/Out, CameraRaise/Lower; default unbound). Six new
DebugVM mirror properties forwarding to CameraDiagnostics so the
upcoming "Chase camera" DebugPanel section can drive them live.

Also folds in four small cleanups from the Task 4 code review:
- Both CameraDiagnostics-mutating tests in CameraControllerTests now
  use try/finally save/restore (consistency with Task-3 follow-up B)
- Drop unused `using System.Numerics` from CameraControllerTests
- Reword the XML doc on CameraController.Active to explain WHY both
  cameras are held simultaneously (flag flip takes effect on the
  next Active access without re-entry) rather than restating the
  getter logic

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

src/AcDream.App/Rendering/CameraController.cs

@@ -11,11 +11,11 @@ public sealed class CameraController
     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"/>.
+    /// The renderer-facing active camera. Both the legacy and retail
+    /// chase cameras are held simultaneously so that flipping
+    /// <see cref="CameraDiagnostics.UseRetailChaseCamera"/> takes effect
+    /// on the very next access to this property — no re-entry required,
+    /// no notification mechanism, no stale state.
     /// </summary>
     public ICamera Active
     {
@@ -53,9 +53,8 @@ public sealed class CameraController
     }
 
     /// <summary>
-    /// Enter chase mode with both candidate cameras. Both are held;
-    /// <see cref="Active"/> picks based on
-    /// <see cref="CameraDiagnostics.UseRetailChaseCamera"/>.
+    /// Store both cameras simultaneously; <see cref="Active"/> picks
+    /// between them per-read via the flag — no re-entry needed on flip.
     /// </summary>
     public void EnterChaseMode(ChaseCamera legacy, RetailChaseCamera retail)
     {