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/visual_targets.json

@@ -48,6 +48,18 @@
         "Art/reference/north-chinese/01-grauman-s-chinese-theatre-by-carol-highsmith.jpg"
       ],
       "expect": "The two north-side buildings (TCL Chinese Theatre, Madame Tussaud's) seen from across Hollywood Boulevard, full height in frame, daylight"
+    },
+    {
+      "name": "demo-third-person",
+      "shot": "Screenshots/demo/review-demo-third-person.png",
+      "size": [
+        1920,
+        1080
+      ],
+      "references": [
+        "Art/reference/hollywood/el-capitan-facade-day-wikimedia-2009-10-27.jpg"
+      ],
+      "expect": "Third-person view of the player body standing on the north sidewalk, full figure visible from behind, the hero facade across the street, daylight"
     }
   ]
 }