Felling trees inappropriately creates dirt squares

# Describe the bug

Logs from a felled tree create dirt squares under themselves inappropriately overwriting other types of terrain. For instance, a tree growing on top of a LMOE shelter can fall on top of stairs and other trees creating dirt squares.

# Steps To Reproduce

1. Find an LMOE shelter or other raised structure with a tree on top
2. Fell the tree so that it falls on stairs/other trees
3. Chop the resulting logs and notice that dirt squares remain

# Expected behavior

A felled tree should not change terrain under itself, especially not create walkable terrain.

# Screenshots


# Versions and configuration

- OS: Arch Linux
- Game Version: O.E-2
- Graphics version: Tiles
- Ingame language: English
- Mods loaded: Disable NPC Needs, No Rail Stations, sees-player icon (retrodays), Bionic slots, Alternative map keys

Start by reproducing the LMOE shelter scenario described in the issue: fell a tree onto stairs or another tree, then chop the logs. Trace the terrain changes caused by tree falling and log removal; done means preserving the underlying terrain without leaving dirt or creating walkable squares.

Written by the indexing model from the issue text.