fix: chase targets without a leash and evade home after 30s out of combat

- Remove the spawn-field leash from Creature.Move so creatures follow their target anywhere within chase range, replacing the edge hold that let players pin creatures in place

- Fold chase range into Creature.CanPursueTarget so targets beyond it are neither kept nor re-acquired, and drop the now-unreachable range branches in Move

- Stamp Creature.LastCombat on every chase step, chase jump and attack; clear it on respawn

- Hold displaced creatures in Reseting until 30 seconds after LastCombat, still scanning for targets, before they return home

- Evade on the first return step: restore full life with a MsgUserAttrib life update, pursue nothing, and fail Combat.ValidTarget until the return field is released

- Rewrite CreatureLeashTests and extend CreatureBehaviorTests for the chase, the 30 second delay, chase range and evading

- Update the monster characterization and pathfinding docs