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/render_screenshots.sh

@@ -39,8 +39,11 @@ run_cmdlet ue_build_review_map.py "5/6 stage the demo shot cameras"
 tail -3 /tmp/ue-review-map-result.txt 2>/dev/null || true
 
 echo "== 6/6 capture the demo map =="
-./Engine/Binaries/Linux/UnrealEditor-Cmd "$PROJ" /Game/Hollywood/DemoMap \
-  -game -windowed -ResX=1920 -ResY=1080 -nopause -nosplash -stdout
+# Absolute engine path: this step runs from the repo, not from $ENGINE, and a bare
+# ./Engine/... failed with "No such file or directory" while every commandlet above
+# succeeded through run_cmdlet's cd. -thirdperson so the review shows the body.
+"$ENGINE/Engine/Binaries/Linux/UnrealEditor-Cmd" "$PROJ" /Game/Hollywood/DemoMap \
+  -game -windowed -ResX=1920 -ResY=1080 -nopause -nosplash -thirdperson -stdout
 
 echo "== collect the demo screenshots =="
 mkdir -p "$REPO/Screenshots/demo"