Moves the four cross-reference / content sanity-check
handlers (--check-zone-refs, --check-zone-content,
--check-project-content, --check-project-refs) out of
main.cpp into a new cli_check.{hpp,cpp} module. Each goes
deeper than --validate (which only checks open-format file
presence) — verifying that quest NPC IDs resolve to
creatures.json entries, model paths resolve to on-disk files,
spawn positions sit inside the zone's tile bounds, etc.
main.cpp shrinks by 509 lines (2,807 to 2,298). All four
preserve --json output for CI pipelines.
