After several manual steps for installing Signal Desktop (download a keyring, install it in the system, add a repo signed by that keyring, and *then* update and install the package) I'm now onto trying to use signal-cli.
It turns out that my options for signal-cli are:
1. Use a newer or different operating system, because it needs a cutting-edge version of Java (JRE 21) while my OS only has version 17; or
2. Use the native build, which requires a newer *processor*, lord only knows why.
This is some real bullshit, Signal.