[FEATURE] Add a monster

Add a basic monster, that walks straight and turns around when it hits a wall. During time travel, it must go the same path it went before, backwards, including "falling" pack up on platforms where it fell down before.

Start by reading the existing time-travel and platform-collision behavior to understand how movement history is represented. Done means a basic monster walks straight, reverses at walls, and retraces its prior path backward during time travel, including falls and returns to platforms.

Written by the indexing model from the issue text.