perf: use sub-sampling for path_finding

[otto-link]

Improve path_finding performance by supporting sub-sampling of the input terrain.

The main application is flow_fixing_mst, where path finding does not always require full-resolution terrain data.

• Allow path finding to operate on a lower-resolution representation.
• Preserve the resulting paths when mapped back to the original resolution.
• Expose the sub-sampling factor where appropriate.
• Evaluate the performance/quality trade-off for flow_fixing_mst.

Goal: significantly reduce path-finding cost while preserving the overall structure of the resulting network.