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.UI.Abstractions/Input/InputAction.cs

@@ -262,4 +262,14 @@ public enum InputAction
     /// <summary>Fly-camera descend (Ctrl) — only meaningful while fly camera
     /// is active. K.1b binds it to ControlLeft; K.1c may rebind.</summary>
     AcdreamFlyDown,
+
+    // ── AcdreamCameraCommands ─────────────────────────────
+    /// <summary>Camera zoom in (held key, integrates Distance−= adjSpeed·dt). Default unbound.</summary>
+    CameraZoomIn,
+    /// <summary>Camera zoom out (held key, integrates Distance+= adjSpeed·dt). Default unbound.</summary>
+    CameraZoomOut,
+    /// <summary>Camera raise (held key, integrates Pitch+= adjSpeed·dt·0.02). Default unbound.</summary>
+    CameraRaise,
+    /// <summary>Camera lower (held key, integrates Pitch−= adjSpeed·dt·0.02). Default unbound.</summary>
+    CameraLower,
 }