Being pushed up the stairs while moving up the stairs sends you two z levels up instead of one

## Round ID:

Some Tycoon round a week or so ago

## Testmerges:
Not relevant

## Reproduction:
1. Invite a friend to help test
2. Stand on the topmost tile of the staircase
3. Time it so the mech pushes you the same moment you walk up the stairs
4. End up in the empty space z level (one above the top deck of the ship)

Start by reproducing the staircase collision using the four listed steps, with a friend and a mech, and confirm the player reaches an empty z level instead of the expected level. Trace the movement and push interaction involved in walking up the stairs at the same moment as the push; done means the player moves only one z level and the reproduction no longer reaches the empty space above the ship.

Written by the indexing model from the issue text.