screensaver-proxy: Activate the screensaver for logind Lock

[radkawar]

Watch logind's Lock signal for this daemon's own login session and activate the screensaver through the existing cinnamon-screensaver-command --lock helper. The helper preserves custom screensaver behavior. The listener belongs in the persistent proxy: cinnamon-screensaver normally exits after 30 seconds idle and cannot receive the signal then. Stop removes the subscription.

Native testing: Linux Mint 22.3 Cinnamon X11, golden KVM guest admitted through the normal lab FIFO. Built an unmodified 6.6.2 daemon package and the same patch backported to 6.6.2, using dpkg-buildpackage -b -us -uc -j2. With the screensaver's LockedHint patch from linuxmint/cinnamon-screensaver#521 installed, waited 35 seconds after each fresh graphical login and confirmed the screensaver bus name was unowned. Baseline: loginctl lock-session left LockedHint=no. Patched: the same request activated the screensaver (GetActive=true, LockedHint=yes); logind unlock returned both to false/no.

AI disclosure: Developed with AI assistance. The native baseline comparison described above was executed.

[mtwebster]

This is handled in Cinnamon now for the upcoming release.

This daemon's main purpose is to forward inhibitors using the freedesktop screensaver api to Cinnamon's session manager.

[radkawar]

Thanks for the explanation. I will keep the tested Mint 22.3 compatibility backport fork-only rather than pursue this for the upcoming Cinnamon release.