feat(editor): add --gen-mesh-training-dummy sparring prop

66th procedural mesh primitive. Combat training dummy from
axis-aligned boxes:

  • vertical post from ground to baseH (the stand)
  • cubic torso block stacked on top of the post
  • horizontal cross-bar arms at upper-third torso height
    spanning armSpan along the X axis
  • optional head cube above the torso (set headSize=0
    for a headless training-block style)

Pairs with --gen-mesh-anvil / --gen-mesh-workbench /
--gen-mesh-fence for sparring grounds, militia drill
squares, training yards, weapon-master compounds. Useful
for any "things-to-hit" set dressing where animated NPCs
would target a static prop.

Watertight under weld (verified 72 manifold edges, 0
boundary, 0 non-manifold). Default 1.0 m post + 0.4 m
torso + 0.18 m head reads at adult-human height.

tools/editor/cli_arg_required.cpp

@@ -54,6 +54,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-pergola", "--gen-mesh-chimney", "--gen-mesh-bedroll",
     "--gen-mesh-workbench", "--gen-mesh-crate-stack",
     "--gen-mesh-watchpost", "--gen-mesh-water-trough",
+    "--gen-mesh-training-dummy",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",
     "--gen-mesh-mailbox", "--gen-mesh-tombstone", "--gen-mesh-crate",