You can't take a powerloader up a flight of stairs

## Testmerges
#12174: 'Yautja Armor Sets' by realforest2001 at commit 08d99ee337
#11698: 'SHARP Resharpening: Design Improvements' by MPhonks at commit 35ee2cf090
#12480: 'Sneaky Hijack Edgecase + Transmute Fixes' by Drulikar at commit be2730f9cf
#12519: 'Revert turf signal preservation + open_space tweaks' by Drulikar at commit 6d6bd7f31f
#12542: 'Xeno defensive grab' by Drulikar at commit 71a0cb6906
#12585: 'stops traps going under jungle bushes' by realforest2001 at commit cf176673f7
#12573: 'Opensearch query builder' by fira at commit b9777a1272
#12626: 'Refurbished Combat Information Centre (2) - USS Almayer' by Steelpoint at commit c167971a27
#12627: 'Tyrargo Rift (2) - Various Alterations' by Steelpoint at commit 5eb283de05
#12658: 'Fix/UI: Ahelps Over-sharing + Ticket Panel tweaks' by MistChristmas at commit 086d3db1eb
#12623: 'refactor: Require all overrides of Crossed to call parent' by boskoramen at commit 3113bb74e2
#12667: 'Toggle Melee Blocking' by realforest2001 at commit debb9fbca1
#12182: 'Multiz projectile balance' by cuberound at commit 864049540e
#12693: 'Fort McNeil/Abyssal Combo Map [TM ONLY] [MDB IGNORE]' by Steelpoint at commit 9f3ce4fb87
#12706: 'MapGrids: QuadTrees Replacement' by fira at commit 31e22a81dc

## Round ID
35485

## Description of the bug
I tried taking a powerloader up a flight of stairs

## What's the difference with what should have happened?
I should have been able to take a powerloader upstairs

## How do we reproduce this bug?
1) Get a powerloader
2) Go to the alamo stairs
3) Try walking up them

## Attached logs
```
N/A
```

## Additional details
- Author: Warfan1815
- Admin: Cark
- Note: None

Reproduce the issue with a powerloader at the Alamo stairs, following the listed steps. Trace the movement handling for powerloaders and stairs, then verify that a powerloader can walk upstairs without breaking other movement behavior.

Written by the indexing model from the issue text.