Prevent monster npc collisions

[kevingranade]

Summary

SUMMARY: Bugfixes "Prevent NPC duplications when climbing stairs and colliding with a monster."

Purpose of change

Fixes #24466
It turns out you could end up occupying the same square as a monster if zlevels were enabled and you climbed stairs with a monster standing on the destination.
Additionally, when occupying the same square as a monster and attempting to swap places with a NPC, the swap would apparently succeed, but the NPC would exhibit a number of errors.

Describe the solution

This fixes both issues. First the swap fails if either target is occupying the same square as a different creature. Second the stair code now detects when destination squares are occupied and avoids them.

[Night-Pryanik]

Maybe replace the debug message for the stairs case with a more user-friendly message like "You cannot go upwards/downwards as there is other creature on the way"?

[kevingranade]

There isn't a debug message for failing to climb stairs, just for the "should never happen" position swap when one of the swapees is on the same square as another creature.

There's a much more involved overhaul to this function that would be capable of giving that feedback, but right now I'm turning data structure corruption (a NPC occupying the same square as a monster) into a failed action.

[Night-Pryanik]

Then what happens now if player is trying to climb stairs with creature occupying the same tile? Silent fail?

[kevingranade]

Of course not, it does something much more sensible...
There is a sheer drop halfway down. Jump? (Y/N)
... Ok that's not particularly sensible.