Implements load_zone() and load_zones() functions to parse zone
definitions from TOML files. Wires zone loading into server startup
to register all zones from content/zones/ directory. Updates player
zone lookup to use the registry instead of hardcoded overworld check.
Includes tavern.toml as first hand-built interior zone (8x6 bounded).
Implements a module-level zone registry for looking up zones by name.
Includes register_zone() and get_zone() functions with comprehensive
tests covering single/multiple zones, unknown lookups, and overwrites.