Monsters cannot fall if tile below is occupied

### Describe the bug

Zombies are traversing on thin air. Without falling down.

### Attach save file

[Norman-trimmed.tar.gz](https://github.com/user-attachments/files/21681772/Norman-trimmed.tar.gz)

### Steps to reproduce

See flying zombie.
Wait a few turns.
Notice "the zombie falls down a level! The zombie climbs to its feet!"
See "climbing to its feet" apparently means teleporting up a level.
Sometimes it stays down, but this happens with other zombies too.

Spawn extra zombies to observe more reliably.

### Expected behavior

Zombies shouldn't fly. Because that's unrealistic?

### Screenshots

### Versions and configuration

- OS: Windows
- OS Version: 10.0.26100.4652 (24H2)
- Game Version: cdda-experimental-2025-08-04-0332 [64-bit]
- Graphics Version: Tiles
- Game Language: System language []
- Mods loaded: [
Dark Days Ahead [dda],
Disable NPC Needs [no_npc_food],
Portal Storms Ignore NPCs [personal_portal_storms],
Stats Through Kills [stats_through_kills],
Translate Complex Dialogue [translate_dialogue],
Tamable Wildlife [Tamable_Wildlife]
]

### Additional context

I know my version is out of date but I didn't see anything in the changelog that would have fixed this.

Edit: It seems monsters can't fall if there's one beneath them, rather than just being shoved to the side. Cool, but I don't think zombies are coordinated enough to hold each other up.

Start with the attached Norman-trimmed.tar.gz save file and follow the listed reproduction steps, including spawning extra zombies. Trace the monster falling and recovery behavior involved when another monster occupies the tile below. Done means zombies no longer remain airborne or teleport up a level after falling.

Written by the indexing model from the issue text.