Boiler can exploit multi-z setup

## Testmerges
#10883: 'Blocks tiles you cannot view while looking up' by Git-Nivrak at commit 5d0d58b9f6
#10926: 'Medals Overhaul' by realforest2001 at commit 988eb723b9
#10999: 'Fix of various plane related issues' by Blundir at commit f85a7153c4
#11136: 'Coms Rolling Encryption & Punchcards' by Drulikar at commit 854048df10
#10954: 'Liquid fuel decals now ignite & Toxic no longer destroys weeds' by Drulikar at commit 17721b78f3
#11139: 'Synthetic Loadout Options' by realforest2001 at commit 7eacd56c98
#10611: 'White Antre Research Facility - Ground Map' by Steelpoint at commit ae4031871b
#10696: '[code bounty] Sentinel rework' by cuberound at commit f9d0bdb4a0
#11008: 'Civilian-nization of Researchers, Complete Redesign of the Research Wing/Containment' by LC4492 at commit 5aa8620aa8

## Round ID
30990

## Description of the bug
With the multi-z boiler now working across Z levels you can now use boiler to shoot "over" walls/doors/obstacles using an upper multi-Z level. This may either be a bug or something that needs to be decided on at a moderation level if it is an exploit (and then clarified to the players).

## What's the difference with what should have happened?
Like with bullets, shooting either up or down a Z-level should have the bullet be on that level. But the boiler is special, because the boils propagate across Z-levels with a slightly smaller radius (-1 tile). But now, a boiler can aim "in the air," over walls and hit boils across any obstacle as long as there is no multi-z floor above that tile. This is logical if your target is on the Z-level you are trying to hit but it creates a weird scenario where shooting the empty space up a Z-level can just shoot over obstacles.

## How do we reproduce this bug?
1) Be boiler
2) Find an empty space above a Z-level (White Antre has a big, empty space)
3) Create walls/cades/obstacles
4) Look up a Z-level
5) Aim at the tile in the upper Z-level above a target
6) Boil will go over all obstacles (as it is now in the upper Z-level, its empty) and the area will be over the target.

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

## Additional details
- Author: WW3_slayer
- Admin: Detective Google
- Note: None