Moves the recently-added composite-prop mesh handlers (rock,
pillar, bridge, tower, house, fountain, statue, altar, portal,
archway, barrel, chest) into their own translation unit. These
12 handlers were the most contiguous block of similar-shaped
mesh code in main.cpp.
Other mesh handlers (--gen-mesh dispatcher, fence, tree, grid,
stairs, disc, tube, capsule, arch, pyramid, from-heightmap,
textured) still live in main.cpp and may be migrated in
subsequent batches.
main.cpp drops 24,270 → 22,681 lines (-1,589). Behavior
verified by re-running rock/chest/archway/fountain.
