Is your feature request related to a problem? Please describe.
Spectrum's progression is fairly complex, so I would like to give players small hints where there might be something interesting to discover, still.
What would be perfect is a small icon marking the entry that draws attention onto it, that renders until the player found what was there to find.

Describe the solution you'd like
Back when using Patchouli, entries supported a tag called turnin, which gave entries an orange dot until an advancement was completed. See the written book entry here:

With Conditions, though, Modonomicon can go quite a bit more fancy instead of just having an advancement check! Maybe even allowing for different dot shapes/colors, like entries support different frame types?

I like this!
Let's brainstorm a bit how this can be implemented.

• Entries get an optional property that determines the "marker" texture
• Entries get a "marker condition". If a condition is present and it is not yet fulfilled, the marker is shown
• Render marker only if the entry is already unlocked (for now, see discussion below )
• Marker condition unlock handling needs to be added to the existing unlock state handling

Anything obvious/big I am missing?

Looks pretty all-emcompassing to me. That should be pretty much everything required.

The only thing that comes to mind is when to render the marker. I feel rendering only when the entry is unlocked ia the most obvious take. Might there be reasons when one want to render it when the entry is already visible, but not yet unlocked? (I got nothing)

Hmm that is a good question.
I think we can start with rendering it when unlocked (because only then the player can open it to investigate I suppose), but if someone has a use case for locked it should be pretty trivial to add