Let OWE own video backgrounds while it runs

The background plugin now watches for the OWE daemon socket. While OWE is
running, the desktop yields video playback to it and the shell keeps
stills. The lock screen keeps its own playback.

This lets Omarchy cooperate with OWE without OWE editing shell.json, so
the engine can ship as a package.

Add a package-list note that owe-wallpaper-engine must be added once it is
packaged.

test/shell.d/video-background-test.sh

@@ -87,6 +87,15 @@ assert(
   'video switches bypass the image-only reveal stack and use the durable background path'
 )
 assert(backgroundQml.includes('BackgroundMedia {') && lockQml.includes('BackgroundMedia {'), 'desktop and lock screen share video-capable media rendering')
+assert(
+  backgroundQml.includes('property bool oweActive: false') &&
+    backgroundQml.includes('/owe/owed.sock') &&
+    /deferVideo: root\.oweActive/.test(backgroundQml) &&
+    /property bool deferVideo: false/.test(mediaQml) &&
+    /active: root\.path !== "" && root\.video && !root\.reloading && !root\.deferVideo/.test(mediaQml) &&
+    !lockQml.includes('deferVideo'),
+  'the desktop yields video backgrounds to OWE while it is running, and the lock keeps its own playback'
+)
 assert(
   lockQml.includes('source: wallpaper.video ? null : wallpaper') &&
     lockQml.includes('visible: !wallpaper.video') &&