ShutdownTestActivity

Developer utility — injects a synthetic ShutdownLifecycle task so the shutdown-progress UX can be exercised end-to-end without a real subsystem retrofit.

Detailed description

UI: an Enable checkbox, a duration slider (1–60 seconds), and a read-only status line previewing what the next quit will do. When enabled, a process-singleton probe registers a task on `beginShutdown` that pretends to do work for the configured number of seconds, with live percent/ETA updates, and self-deregisters on natural completion or forceCancel.

The probe is installed exactly once per process (on first activity construction). Disabling the checkbox just flips the "armed" flag — the probe stays in the manager's registry but becomes a no-op.

Spec: content/doc/internal/specs/shutdown-lifecycle/

See also ShutdownLifecycle, ShutdownProgressActivity.