[Feature] Respect Layer Window Rule (maybe a bug?)

Is your feature request related to a problem? Please describe.
It happens in a few programs, but the most prominent and irritrating is the Picture-in-Picture of Firefox. It doesn't work by default (under Wayland) in KDE but one can set the "Layer" in "Window Rules" to either "On-Screen display" or "Overlay". I also tried others (like "popup" etc.)

Describe the solution you'd like
Either respect window layers by default, or add the ability to add it as a rule in the Karousel-settings rules.

Describe alternatives you've considered
I have tried adding a rule like so:

{
    "class": "Firefox",  # (also tried "firefox firefox")
    "caption": "Picture-in-Picture",
    "tile": false
}

but for some reason it won't work.

Start by tracing Karousel's existing settings-rule handling and how KDE/KWin exposes window layers; reproduce the Firefox Picture-in-Picture case under Wayland. Clarify whether default layer handling or a configurable rule is in scope, then verify the chosen behavior with the supplied Firefox rule scenario.