feat(editor): add --gen-mesh-archery-target training prop

70th procedural mesh primitive. Round-faced archery target on
a 2-post stand:

  • face: closed cylinder along the Z axis (flat ±Z caps face
    the archer line, side wall is the rim) of radius faceR
    centered at height postH
  • stand: 2 vertical posts at the bottom, sized to reach
    from ground to the bottom of the face, with a horizontal
    cross-beam joining them just below the face for rigidity

Concentric scoring rings live in texture space, not geometry —
the cap discs are the natural canvas for a separate target-
ring texture (or use --gen-texture-rings / --gen-texture-
starburst on top).

Pairs naturally with --gen-mesh-training-dummy /
--gen-mesh-fence for sparring grounds, training yards,
militia drill squares, mid-summer fair scenes.

Watertight under weld (verified 198 manifold edges, 0
boundary, 0 non-manifold).

tools/editor/cli_arg_required.cpp

@@ -56,6 +56,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-watchpost", "--gen-mesh-water-trough",
     "--gen-mesh-training-dummy", "--gen-mesh-hitching-post",
     "--gen-mesh-outhouse", "--gen-mesh-forge",
+    "--gen-mesh-archery-target",
     "--gen-camp-pack", "--gen-blacksmith-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",