fix(launcher): recover a stale client tree without restarting Darling

Darlingserver normally stays alive after the Riot Client or game is closed.
If the client window disappears but Riot/League/CEF processes remain,
restarting via run.sh reuses a half-dead runtime and can fail to open a
window.

Add an opt-in RCLOL_RECOVER_STALE_CLIENT=1 preflight. When a darlingserver is
alive, no game is running, no managed client window is visible and stale
client processes belong to this prefix, stop only that client/CEF tree.
Preserve darlingserver, launchd/mldr, sockets and .init.pid, then continue
with the normal product launch.

The helper fails closed when X11 is unavailable, never touches a running game
or a visible client window, and does not patch client binaries, CEF or assets.
It saves the Darling bootstrap, but not Riot Client/CEF startup; a full runtime
restart remains the fallback if no window appears.

.github/workflows/source-check.yml

@@ -31,3 +31,4 @@ jobs:
       - run: python -m unittest discover -s tests -p 'test_metal_device_info.py'
       - run: python -m unittest discover -s tests -p 'test_exec_guest.py'
       - run: python -m unittest discover -s tests -p 'test_close_league_client_in_game.py'
+      - run: python -m unittest discover -s tests -p 'test_recover_stale_client.py'