Body step 2.4: landing, movable ground, playground fixes

- legs brake landings at 4.5 g; vertical support is a rate-limited servo
  (no launch when stepping onto a mantled ledge)
- ground control pushes back on movable ground (seesaw tips under the player)
- playground hinges use absolute paths (joint paths resolve from the joint)
- big props grabbable via surface query; Reset props keeps playground pieces
- spike: landing, seesaw, playground joints/grab, person-like wall climb

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/PropShop.cs

@@ -34,8 +34,11 @@ public partial class PropShop : Node3D
 
     public void Respawn()
     {
-        foreach (Node node in GetTree().GetNodesInGroup("prop"))
-            node.QueueFree();
+        // Only the table's own props (the playground's dynamic pieces are props too — they reset
+        // in place via Playground.Reset).
+        foreach (Node node in GetChildren())
+            if (node is RigidBody3D && node.IsInGroup("prop"))
+                node.QueueFree();
         foreach (PropSpec spec in Specs)
             SpawnProp(spec);
         GD.Print("PropShop: table reset");