Moves the three mesh-PNG bridge handlers (--displace-mesh,
--gen-mesh-from-heightmap, --export-mesh-heightmap) out of
main.cpp into their own translation unit. These three are
distinct from the gen-mesh-* primitive generators in that
they read or write external image files rather than synthesize
geometry from parameters alone.
main.cpp drops 21,061 → 20,741 lines (-320). Behavior verified
by re-running gen-mesh-from-heightmap → validate-wom → and
displace-mesh on a fresh plane.
