Populate lakes with boats

Especially around the bodies of water in Seattle, there are sailboats and kayaks moving around. It would be interesting to simulate these to add flair. This is low priority in the scope of the project, but I think it could make a potentially fun starter project and does have value:

- any enhancement to the game's visuals can better engage people and help them understand changes to the map in context
- the implementation could be reused for simulating pedestrians moving through non-linear plaza areas
- in places with significant ferry-based public transit, potentially water-based movement is important to model (although ferries would likely be snapped to linear routes)

I have some thoughts about how to come up with an efficient event-based, parametric-over-time implementation. I think it should live outside of `sim`. Some of the first tasks, geometry-focused:
- picking random points in an area's polygon and generating a path between them that stays within the polygon
- interpolating boat position along that PolyLine path
- solving for collisions between different boats, chopping up the path into pieces, and adding a "wait for the other boat" phase