Removes dependency on global players dict for spatial queries by using
Zone.contents_at() for spectator lookup. Makes _world local to run_server()
since it's only used during initialization to create the overworld Zone.
Updates test fixtures to provide zones for spatial query tests.
Helper function sends messages to all players at the same x,y coordinates
as the source player, skipping the source player themselves. Used for IF
spectator broadcasting.
Tests verify:
- Spectators at same location see IF output with player name header
- Spectators at different locations see nothing
- Game start intro broadcasts to spectators
- broadcast_to_spectators skips the playing player
- Multiple spectators all receive messages
Tests currently fail as broadcast_to_spectators not yet implemented.
