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.

docs/SETUP.md

@@ -78,6 +78,14 @@ opts in to Hyprland-only recovery of a default-prefix Riot session with no
 visible window; it is off by default. See [troubleshooting](TROUBLESHOOTING.md#riot-exits-and-league-reports-that-it-lost-its-connection)
 for the observed failures, recovery guards and NSURLSession limitations.
 
+`RCLOL_RECOVER_STALE_CLIENT=1` opts in to partial runtime recovery before
+launch. If `darlingserver` is alive, no game is running, no managed client
+window is visible and stale Riot/League/CEF processes remain, the launcher stops
+only that client tree and reuses the runtime. It preserves `darlingserver`,
+launchd/mldr, sockets and `.init.pid`; if X11/Xwayland cannot be inspected it
+does nothing. See
+[troubleshooting](TROUBLESHOOTING.md#client-remains-running-without-a-window).
+
 `RCLOL_FILTER_HOST_ENV=1` opts in to removing host desktop-session variables before
 the guest starts: `WAYLAND_DISPLAY`, `XDG_SESSION_*`, `XDG_CURRENT_DESKTOP`,
 GNOME/KDE/GDM, systemd, SSH agent, terminal, toolkit (`QT_*`, `GTK_*`, `GDK_*`),

docs/TROUBLESHOOTING.md

@@ -149,6 +149,15 @@ ownership or window information cannot be read. A hidden background client or
 download is restarted when recovery is enabled. Other desktop environments are
 not supported by this helper; recovery is off by default.
 
+For a live Darling runtime with no visible client window but stale Riot/League/
+CEF processes, `RCLOL_RECOVER_STALE_CLIENT=1` performs a partial recovery before
+the next launch. It keeps `darlingserver`, launchd/mldr, sockets and `.init.pid`,
+stops only the stale client process tree, and then lets the normal launcher start
+the product again. It does nothing while a game process is running or when a
+managed client window is visible. With X11/Xwayland unavailable it fails closed.
+This saves the Darling bootstrap but not the Riot Client/CEF startup itself. If
+no window appears afterward, use a full runtime shutdown/restart.
+
 ### Client window stays black after a match
 
 The League client keeps its Chromium/CEF UI (`LeagueClientUx` and its helpers)