Moves the two content-density audit handlers
(--info-zone-density, --info-project-density) out of main.cpp
into a new cli_info_density.{hpp,cpp} module. Both count
creatures/objects/quests per tile to surface sparse zones
(boring) and over-stuffed ones (frame-rate bombs); the project
variant rolls per-zone numbers into a project-wide table with
per-zone averages for content-pacing reviews.
main.cpp shrinks by 201 lines (7,254 to 7,053). Both --json
output modes preserved for balance-audit pipelines.
