SessiondBridge: handle pushed {"directive":"lock"} on the heartbeat line and
raise the lock surface. Unknown directives are console.error — a silent drop
leaves the daemon blanking unlocked when its budget expires.
SessiondPolicy: new singleton, reads/writes DeviceStatePolicy over its own
short-lived connection (not the bridge socket — that EOF is shell-death
detection).
IdleConfig: new "Lock screen (device authority)" section bound to it. SetPolicy
had zero callers, so the old spinboxes moved a JSON file the daemon never read.
Says so in the error colour when sessiond is unreachable.
New file needs a deploy.sh manifest line or the services qmldir fails whole.
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