a collision could shove a unit past the edge of the arena. the next path
search then turned that negative coordinate into a tile index, cast it to
usize and read far off the end of the grid - the panic killed the tokio
task running the battle, snapshots stopped, and the models the client had
already been told about were left frozen and unresolvable. that is the
invisible-unit symptom: the simulation was dead, not the rendering.
the push is clamped to the arena now, and find_path checks that the start
tile is on the map rather than only the goal.
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