[BUG] Radiation can still spread across virtual z-levels

[BogCreature]

Round ID: 1385

Testmerges:

#1702: 'New Rockplanet ruin, Budgetcuts!' by spockye at commit 4be20dd4ae #1774: 'Slice blood bag' by ritorizo at commit 288dee3972 #1778: 'fixes industrial_loading' by spockye at commit 506944099d #1807: 'Make plumbing machines not disconnect when ship is docking/undocking' by antropod at commit 240fce8fbb #1671: 'The Vela-Class, a Gold Robotics/Mining ship' by Bjarl at commit 6f120e382f #1817: 'Map Exporter Overhaul' by MarkSuckerberg at commit 20a50e0634 #1771: 'Cutter Class Pirate Raider' by spockye at commit 98ec3e6f23 #1820: 'ports the exosuit pka from tgstation' by Bjarl at commit 7d8b06c1c7 #1822: 'Shetland Fix' by Bjarl at commit 27bcb8d1d0 #1819: 'Ports TG leg rendering' by MrSamu99 at commit 5182976eb1 #1781: '[Needs TM] The Accelerataning' by MarkSuckerberg at commit 207243cf13 #1780: 'Overmap Expansion 4.2' by MemedHams at commit 0dc96c76fd #1783: 'Botany Balance Pass' by MemedHams at commit 14332aa7b3 #1823: 'Changes Glaive Turfs' by Bjarl at commit 912a5257e6

Reproduction:

Create a fusion reaction with a rad count around 2x10^26 or so and, despite the z-level walls having perfect rad insulation, witness as it spreads to any other ships that share the same hyperspace z-level

[tmtmtl30]

oh god dammit

[AverageUser67]

so thats why the humans were getting cancer