I finally came back and finished part 2. 40-50 seconds on the core i5… 
I updated part 1 to detect loops, ran it again to get the path travelled, calculated all the adjacent spaces, then brute forced it re-running part 1 with an obstacle in every space, counting how many of them resulted in a loop.
Enum.count(candidate_obstructions, fn {x, y} ->
new_obstacles = MapSet.put(patrol.obstacles, {x, y})
new_patrol = %Patrol{patrol | obstacles: new_obstacles}
patrol(new_patrol) == :loop
end)
https://git.adamu.jp/adam/AdventOfCode/src/branch/main/2024/day6.exs
