feat(camera): third-person rig, V toggle, visible body

One pawn, both rigs (ADR-9): a skeletal body hidden in first person, a
collision-tested spring arm, and a toggle that swaps the active camera and the
body's visibility. Shipped default stays first person; -thirdperson starts the
pawn in the third-person rig so the review shot is reproducible, and the
screenshot run passes it.

Three defects found and two fixed:
- The skin material had no SkeletalMesh usage flag, so Unreal silently substituted
  the default grey. That is why the body rendered as clay.
- render_screenshots.sh still called ./Engine/... on this branch (the fix lives on
  the facade branch and never reached main); applied here so the shots generate.
- OPEN: the idle clip is bound and PlayAnimation is called - the run logs the clip
  name - but the body still renders in bind pose, arms out. Recorded with that
  evidence in TODO.md rather than called done.

Shot 5 (demo-third-person) added and documented; camera collision is implemented
but unproven, since the spawn has open space behind it.

Tools/ue_create_input_assets.py

@@ -127,6 +127,10 @@ def build():
     ia_reset.set_editor_property("value_type", unreal.InputActionValueType.BOOLEAN)
     save(ia_reset)
 
+    ia_cam = fresh("IA_ToggleCamera", unreal.InputAction, unreal.InputAction_Factory)
+    ia_cam.set_editor_property("value_type", unreal.InputActionValueType.BOOLEAN)
+    save(ia_cam)
+
     ia_pause = fresh("IA_Pause", unreal.InputAction, unreal.InputAction_Factory)
     ia_pause.set_editor_property("value_type", unreal.InputActionValueType.BOOLEAN)
     save(ia_pause)
@@ -136,7 +140,7 @@ def build():
         (ia_move, "Gamepad_LeftY"), (ia_strafe, "Gamepad_LeftX"),
         (ia_look, "MouseX"), (ia_look, "MouseY"), (ia_look, "Gamepad_Right2D"),
         (ia_jump, "SpaceBar"), (ia_jump, "Gamepad_FaceButton_Bottom"),
-        (ia_reset, "R"), (ia_pause, "Escape"),
+        (ia_reset, "R"), (ia_pause, "Escape"), (ia_cam, "V"),
     ]
 
     mappings = []