Far walks that find their way

The server walks a far WalkTo in straight legs of 24 tiles inside the region's arena (uo_world::walk::waypoint). A leg whose end has no reachable free tile within 6 ends the walk, so a click far beyond a walled city's houses, or a bot's goal across a district, goes nowhere: the onboarding bot (#123) got stuck in Aquais until it followed the guarded highway's polyline itself.

• A coarse route over regions (roads, gates, bridges) for legs, then the arena's A* within each.
• The client predicts the same legs (predict.rs), so the planner must stay shared.

Done when a click on the far side of Aquais (or a WalkTo to a town 500 tiles off) gets there.