How to reference class from Runtime asmdef in Editor asmdef from within a local unity package?

For the life of me, I can’t figure this out.
I have a local unity package with this folder structure:

Runtime/
     HelixGenerator.cs
     Lumic.Music.asmdef
Editor/
    HelixGeneratorEditor.cs
    Lumic.Music.Editor.asmdef (this one only has "Editor" checked in target platform)

Now, this code fails to compile because it can’t find the type (classname is correct, correct namespace imported).

[CustomEditor(typeof(HelixGenerator), true)]
public class HelixGeneratorEditor : Editor

When I inspect the project references in Visual Studio, I don’t see “Lumic.Music” being referenced in “Lumic.Music.Editor”. I don’t find any option anywhere to be able to add that reference.

174063-screenshot-2021-01-11-233527.jpg

Am I missing something?

Add assembly references via Inspector. Just select the asmdef file and add the referenced assembly to the Assembly Definition References list.