fix(unified): make tray quit non-blocking and stop every sidecar descendant

[tnunamak]

Tray Quit or an external SIGTERM could leave the unified reference implementation and console sidecars, including descendants, running after DataConnect terminated.

The first fix routes RunEvent::ExitRequested through one background shutdown path, prevents the first exit request, stops both sidecars concurrently under a ten-second budget, and calls app.exit only after cleanup. The follow-up finding was that a default external signal terminates the Rust process before Tauri reaches RunEvent::ExitRequested or RunEvent::Exit; the supervisor now registers active POSIX process-group IDs and a signal-handling thread sends SIGKILL to every registered process group before restoring default signal termination. The normal menu path keeps its bounded graceful-signal, polling, and force-kill sequence.

Evidence: the Unix parent-signal test starts a grouped sidecar plus descendant, sends SIGTERM to the helper parent, and verifies both processes die; the supervisor suite passes 9/9 runnable tests with one helper ignored, and the unified suite passes 7/7. Full cargo test --all-targets reports 200 passed, 34 failed, and 4 ignored; I did not resolve those failures because they match this checkout's known missing packaged connector/runtime artifacts, including p-queue and patchright. Remaining uncertainty: clippy is still blocked by pre-existing errors in src/commands/oci.rs, and GUI quit behavior was not manually verified because the task brief forbids launching the app.

Assisted-by: AI

[tnunamak]

Folded into #141. Assisted-by: AI