Continues the modularization started in 6c9ab6fa. Moves the four
pack-orchestrator handlers (--gen-zone-texture-pack, -mesh-pack,
-starter-pack, --gen-project-starter-pack) into their own file
following the same handle<Family> pattern.
Side cleanup:
- Consolidated the duplicated --seed flag parser into a single
parseSeedFlag helper
- Consolidated the std::system + > /dev/null wrap into runSilently
main.cpp drops 28,329 → 28,070 lines (-259). Pack family is fully
self-contained (~260 lines), behavior unchanged (verified by
re-running gen-zone-starter-pack and confirming 6 PNGs + 5 WOMs).
