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_help.cpp

@@ -252,6 +252,8 @@ void printUsage(const char* argv0) {
     std::printf("                         Watchpost: tall pole + square platform + 4 corner railing posts (sentry / scout outpost)\n");
     std::printf("  --gen-mesh-water-trough <wom-base> [length] [width] [height] [wallT]\n");
     std::printf("                         Water trough: 4-walled rectangular basin with flat floor (stable / farm / tavern set dressing)\n");
+    std::printf("  --gen-mesh-training-dummy <wom-base> [baseH] [postW] [torsoSize] [armSpan] [armT] [headSize]\n");
+    std::printf("                         Training dummy: post + cubic torso + cross-bar arms + optional head (sparring / drill yard)\n");
     std::printf("  --gen-mesh-table <wom-base> [width] [depth] [height] [legThick] [topThick]\n");
     std::printf("                         Table: flat top slab on 4 corner legs (default 1.6/1.0/0.85/0.10/0.06)\n");
     std::printf("  --gen-mesh-lamppost <wom-base> [poleH] [poleT] [baseSize] [lanternSize] [lanternH]\n");