Unable to push past, shoot, or melee zombie on ramp

### Describe the bug

I've got a zombie stuck on a ramp, with me stuck at the bottom of the ramp. This happened when I attempted to melee attack the zombie from the upper z-level, and instead used the ramp to go down. The zombie does not seem to move until I debug teleport up a z-level.

When standing adjacent to the ramp, the zombie is not visible on either the current or upper z-level. Moving into the ramp flashes the zombie sprite and does not pass a turn. Attempting to melee says there's nothing to melee.

When standing one or more tiles away from the ramp, the zombie is visible on the upper z-level, standing directly on the ramp tile. Attempting to fire a gun shows proper aiming calculations, but even with a 100% hit rate the bullet doesn't do anything. It's possible it's hitting terrain (such as the ceiling above me) or something. Hitting tab to attack with a reach weapon (pipe spear) when one tile away doesn't work either.

As far as I'm aware, I am permanently stuck in the basement of a potter's cottage.

### Attach save file

[Greenhorn-trimmed.tar.gz](https://github.com/user-attachments/files/19014012/Greenhorn-trimmed.tar.gz)

### Steps to reproduce

1. Stand next to the top of the ramp.
2. Spawn an enemy at the top of a ramp.
3. Move into the enemy to perform a melee attack.

### Expected behavior

I initially expected to melee attack the zombie rather than take the ramp. Following that I expected to either be able to take the up ramp or melee attack the zombie.

### Screenshots

_No response_

### Versions and configuration

- OS: Linux
- OS Version: Distributor ID: Linuxmint; Description: Linux Mint 21.3; Release: 21.3; Codename: virginia;
- Game Version: 187cf76 [64-bit]
- Graphics Version: Tiles
- Game Language: System language []
- Mods loaded: [
Dark Days Ahead [dda],
Portal Storms Ignore NPCs [personal_portal_storms],
Disable NPC Needs [no_npc_food]
]

### Additional context

_No response_

Start by loading the attached Greenhorn-trimmed.tar.gz save and reproducing the listed ramp interaction with a zombie. Compare melee, firearm, and reach-weapon behavior from adjacent and distant tiles across the two z-levels. Done means the player can resolve the encounter by attacking or traversing the ramp without becoming permanently stuck.

Written by the indexing model from the issue text.