Bound coastal preference by checked distance and destination progress

[woensug-choi]

Coastal preference could add long bay visits and island loops while making only a small net gain toward a local waypoint. Bound it to 15% extra distance against a shorter checked route to the same exit, require destination-port progress and an approaching final stretch, and retain that shorter water route when scenery is rejected. Required land/depth detours remain available.

Validation: 61 routing and port-approach tests pass. A replay at the recorded western Lieyu approach using real GEBCO 2026 and OSM land shortens the route to the same exit from 7.86 km to 5.20 km. The replay does not reconstruct archived satellite bathymetry or historical native collision grids. English and Korean documentation agree.