Zone TOML files can now define portals using [[portals]] sections.
Each portal specifies coordinates (x, y), a target (zone_name:x,y),
and a label. Optional aliases are supported. Portals are
automatically created and placed in the zone when it loads.
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).