(probably more trouble than it's worth) include the slower walking speed through vines in cost calculation

Just like walking on soul sand, walking through vines slows you down. Include this in cost calculation? I can't imagine how many extra checks it would be, so I'm hesitant. Worth it?

The issue names no files, tests, or entry points. Start by locating the pathfinding cost calculation for soul sand and the handling of vines, then determine how movement slowdown is represented and where coverage belongs. Done means vine movement is included in cost calculation without unnecessary checks, with relevant tests passing.

Written by the indexing model from the issue text.