"Teleporting" when moving to a lower level

### Describe the bug

If a character is standing on the same section of the map with a level down, "Descent stairs" will instantly move them to the level below.

The error does not affect moving to a higher floor.

### Attach save file

N/A

### Steps to reproduce

1. Create a character
2. Go to the one section of the map with the stairs to the lower level
3. Descend stairs

### Expected behavior

Obviously, it's not how the descent is supposed to work.

### Screenshots


### Versions and configuration

- OS: Windows
- OS Version: 10.0.19045.3803 (22H2)
- Game Version: 7b2ac96 [64-bit]
- Graphics Version: Tiles
- Game Language: Русский (92.8%) [ru]
- Mods loaded: [
Dark Days Ahead [dda]
]

### Additional context

_No response_

Start by reproducing the three listed steps in Cataclysm: Dark Days Ahead, focusing on the map section with stairs to a lower level and comparing descent with movement to a higher floor. Trace the lower-level stair handling and verify that descending no longer instantly relocates the character beyond the intended stair transition.

Written by the indexing model from the issue text.