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/character_anim_build.py

@@ -55,6 +55,20 @@ ELBOW = 0.18
 
 PARTS: list[dict] = []
 
+# The .blend stores the raw A-pose bind: `mpfb_character_export.py --pose idle`
+# bakes the arms down only in the exported FBX, not in the file. A clip that
+# animates only swing therefore leaves the arms out in an A — which is exactly how
+# the first third-person render looked. These are the same deltas `IDLE_POSE` uses,
+# applied as a base that every key sits on top of.
+BASE = {
+    "shoulder01.L": (0.0, -0.15, -0.03),
+    "shoulder01.R": (0.0, 0.15, 0.03),
+    "upperarm01.L": (0.0, 0.0, -1.55),
+    "upperarm01.R": (0.0, 0.0, 1.55),
+    "lowerarm01.L": (0.0, 0.0, -0.10),
+    "lowerarm01.R": (0.0, 0.0, 0.10),
+}
+
 
 def log(msg):
     PARTS.append(msg)
@@ -78,7 +92,9 @@ def pose_bones(arm):
 
 
 def key_euler(pb, frame, x=0.0, y=0.0, z=0.0):
-    pb.rotation_euler = (x, y, z)
+    """Key a pose-bone rotation on top of the arms-down base."""
+    bx, by, bz = BASE.get(pb.name, (0.0, 0.0, 0.0))
+    pb.rotation_euler = (bx + x, by + y, bz + z)
     pb.keyframe_insert(data_path="rotation_euler", frame=frame)