Building a #flatpak is insane. You people are kidding, right?
Just specifying the sdk in the #flatpak manifest causes the build to fail. It never even gets to my janky source code.
"""
Dependency Sdk: org.gnome.Sdk master
Installing org.gnome.Sdk/x86_64/master from flathub
error: Nothing matches org.gnome.Sdk in remote flathub
"""

That rocks.
Apparently, to get flatpak-builder to download a *version* of an SDK, you have to provide a "runtime-version," which is totally clear because it says "sdk" right there in "runtime-version."