Mindustry: Phase fabric wall

[Plagueboi]

Platform:Windows

Build: steam build 104.10

Issue: unable to build phase walls onto other walls (like what you can do with most walls)

Steps to reproduce: i was inside nuclear complex making my defences, it was an hour into it and i tried to build a phase wall onto a plastanium one when it would not let me

Link(s) to mod(s) used: none

Save file: 6.msav

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[Anuken]

You can only upgrade walls directly when the newer wall has more health.