Adds --import-obj as the 5th handler in cli_wom_io.{hpp,cpp},
joining its export-side sibling. The handler is the WOM-side
counterpart to --import-wob-obj (which lives in cli_world_io
because it produces WOB) — together they close the OBJ
round-trip story for both single meshes and multi-group
buildings.
main.cpp shrinks by 183 lines (6,786 to 6,603).
Moves the four WOM interchange-format handlers (--export-obj,
--export-glb, --export-stl, --import-stl) out of main.cpp into
a new cli_wom_io.{hpp,cpp} module. WOM is our open M2
replacement; these are the bridge that lets it round-trip
through every external 3D tool — Blender, Three.js, slicers,
CAD packages — so the open format is actually useful.
main.cpp shrinks by 467 lines (9,464 to 8,997). The five WOB
and WHM exporters (--export-wob-glb, --export-whm-glb, etc.)
remain inline for a follow-up extraction.
