App targets fail to link to system Qt6 on Arch, can't find QString::toStdString

[ada-tv]

Trying to build Interface spews a bunch of linking errors with "undefined reference to QString::toStdString() const". No idea what's changed since I last built the Qt6 branch.

I've tried adding the Qt::Core package to Interface's CMakeLists with no result. Neither release or debug builds can link.

Weirdly, this doesn't affect our sub-libraries (which use toStdString plenty and don't have link errors), only the app targets like interface, domain-server, assignment-client, oven, etc.