Add shared room creature instances

[halfdimension]

Adds backend-owned shared creature instances for multiplayer rooms and displays them on the map.

• Adds room creature spawn/list APIs
• Allows only room host to spawn creatures during a RUNNING room game
• Lets all room members read the same shared creature instances
• Adds frontend shared creature markers on the map
• Polls shared room creatures while a room game is running
• Keeps catch behavior, scoring, and concurrency handling out of scope
• Keeps solo gameplay unchanged