Desktop is visible through semi transparent lockscreen

Wayfire version

0.11.0-3c9a7712

GPU / Driver

Device: AMD Radeon 780M Graphics (radeonsi, phoenix, LLVM 21.1.6, DRM 3.64, 6.18.5-arch1-1) (0x15bf)

Describe the bug

A lockscreen with semi-transparent background shows the desktop through it. I was originally going to label this 'Blur effect does not get applied to lockscreen' but reading the lockscreen spec[1] leads me to believe it shouldn't be allowed at all.

To Reproduce

Install swaylock,
create/edit ~/.config/swaylock/config

color=00000022

run swaylock

Expected behavior

I would expect the desktop to be blurred behind the lockscreen, but the spec says it should be blanked opaque or possible background-shell[1]

Screenshots / Videos / Stacktrace

Additional context

Very related to #2911

[1]
https://wayland.app/protocols/ext-session-lock-v1#ext_session_lock_v1

The only surfaces that should be rendered while the session is locked 
are the lock surfaces created through this interface and optionally, at 
the compositor's discretion, special privileged surfaces such as input 
methods or portions of desktop shell UIs.

For a creative interpretation of "portions of desktop shell UIs" we could identify and render only the current background-shell. But this may be more of a challenge than I anticipate.

Reproduce the issue with swaylock configured with color=00000022, then inspect the compositor's session-lock handling and compare its rendering behavior with the ext-session-lock-v1 specification. The work is done when the desktop is not visible while the session is locked, including for a semi-transparent lockscreen, with behavior consistent with the expected lock-surface rules.