feat/PoC: add terrain-aware walker pathfinding

[otto-link]

Add a terrain-walker pathfinding algorithm that generates human-like paths through terrain, rather than a globally optimal shortest path.

Starting from A, the walker:

1. Looks toward B and analyzes the locally visible terrain.
2. Selects a plausible intermediate waypoint, favoring natural features such as valleys, saddles, passes and gentle slopes.
3. Uses Dijkstra to reach that waypoint.
4. Reassesses the terrain from the new position and selects the next waypoint.
5. Repeats until reaching B.

Unlike standard Dijkstra/A*, the walker should not have global knowledge of the terrain. The path emerges from a sequence of local decisions, potentially producing a longer but more plausible trail.

This could be particularly useful for procedural road, trail and settlement generation.