Zones can now define spawn rules in TOML:
- [[spawns]] sections specify mob type, max count, and respawn timer
- SpawnRule dataclass stores configuration
- load_zone() parses spawn rules from TOML
- Added example spawn rules to treehouse zone (squirrel, crow)
This is configuration infrastructure only - actual spawning logic
will be handled by the game loop in a future phase.
Zone(Object) is a spatial area with a terrain grid. Supports
toroidal wrapping and bounded clamping, passability checks,
viewport extraction, and contents_at(x, y) spatial queries.
Zones are top-level containers (location=None) that accept
everything via can_accept().
