Inventory saved as JSON list of thing template names in an inventory
column. Migration adds column to existing databases. load_player_data
returns inventory list, save_player serializes Thing names from contents.
Removed module-level world variable and replaced all world.wrap() calls
with player.location.wrap(). Added Zone assertion for type safety,
matching the pattern in movement.py. Updated tests to remove fly.world
injection since it's no longer needed.
Implements account management with password hashing (pbkdf2_hmac with SHA256)
and constant-time comparison. Includes player state serialization for position
and inventory persistence across sessions.
