feat(launcher): hide the League client UI while a match runs

Add an opt-in alternative to closing LeagueClientUx: hide the managed
LeagueClientUx and Riot Client X11 windows while the game process runs and
show them again when it exits. The client stays open; the game window is
never touched.

Detection is attributed to the launcher prefix through __mldr_sockpath and
fails closed when proc, ownership or window state is unreadable. The helper
requires X11/Xwayland, restores the window on end, signal or error, and is
mutually exclusive with RCLOL_CLOSE_CLIENT_IN_GAME=1.

Document the flag, compliance scope and limits. A local 30-second test on an
i5-3470/RX 550 reduced summed client/CEF CPU from 120.7% to 93.4%; this is a
CPU-bound-host measurement, not a universal FPS claim.

No client binaries, CEF, assets or anti-cheat are modified.

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_HIDE_CLIENT_IN_GAME=1` opts in to hiding the managed `LeagueClientUx`
+and `Riot Client` X11 windows while a match runs, then showing them when the
+game process ends. It keeps the client open and is mutually exclusive with
+`RCLOL_CLOSE_CLIENT_IN_GAME=1` (close takes precedence). It is off by default,
+requires X11/Xwayland, and does not patch client binaries, CEF or the game. See
+[the troubleshooting measurement](TROUBLESHOOTING.md#keep-the-client-open-but-hide-it-during-a-match)
+for the measured CPU effect and limitations.
+
 `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

@@ -192,6 +192,55 @@ observation, without waiting for a match:
 python3 tools/close-league-client-in-game.py --once --dry-run
 ```
 
+### Keep the client open but hide it during a match
+
+`LeagueClientUx` can consume a noticeable amount of CPU while the client stays
+open during a match. An opt-in alternative to closing it hides the managed
+client window instead:
+
+```sh
+RCLOL_HIDE_CLIENT_IN_GAME=1 ./scripts/run.sh
+```
+
+The launcher detects when a match starts, unmaps the `LeagueClientUx` and
+`Riot Client` X11 windows, and maps them again when the game process ends. The
+client is not closed, the League backend and Riot client keep running, and the
+game window is never hidden. `RCLOL_CLOSE_CLIENT_IN_GAME=1` takes precedence
+when both flags are set.
+
+The helper is off by default and window-management only: it does not patch
+client binaries, CEF, anti-cheat or DRM. It attributes the game to this prefix
+through the loader's `__mldr_sockpath`, and fails closed when ownership, proc
+state, environment or window information cannot be read. It requires an X11 or
+Xwayland desktop; on a native Wayland window manager without Xwayland it cannot
+find the windows.
+
+On a CPU-bound host the effect appears after CEF has been hidden for a few
+seconds. In a local 30-second test on an i5-3470 with a Radeon RX 550, hiding
+`LeagueClientUx` reduced the summed CPU of the matched client/CEF processes from
+120.7% to 93.4% (about 23%, or 27 percentage points, of one core); showing the
+window returned it to 119.9%. This is not a universal FPS or percentage claim:
+a game that is GPU-bound, a client with no active UI work, or a different
+desktop may show less benefit. The hidden window can also need a moment to
+repaint after it is shown.
+
+`RCLOL_CLOSE_CLIENT_IN_GAME=1` remains the more aggressive option when in-client
+chat during the match is not needed. Configure one of the two, not both:
+
+```sh
+# Keep the client open, hide its windows while playing
+RCLOL_HIDE_CLIENT_IN_GAME=1 ./scripts/run.sh
+
+# Or close the client UI while playing
+RCLOL_CLOSE_CLIENT_IN_GAME=1 ./scripts/run.sh
+```
+
+Diagnose the match detector without changing any window:
+
+```sh
+python3 tools/hide-league-client-in-game.py --once
+```
+
 ### Riot reports NO INTERNET FOUND while the host is online
 
 Older base runtimes ship Google DNS servers (`8.8.8.8` and `8.8.4.4`), which