fix(launcher): close the League client UI while a match runs

The client does not come back properly after a match when its own "Close client
during game" option is enabled. The UI (LeagueClientUx and its helpers) is a
Chromium/CEF instance and, under Darling, the pre-match instance keeps running:
the client the backend starts when the match ends is a second CEF instance on the
same --user-data-dir, and its window never paints. Its DOM and audio work and the
window stays black; closing the client by hand during a match leaves the same
state for the same reason.

RCLOL_CLOSE_CLIENT_IN_GAME=1 opts in to closing that UI from the host, and only
while a match runs, so the client that appears afterwards is the only instance.
The UI also competes with the game for CPU: on a four-core host with a Radeon
RX 550 a practice match reported roughly 120 FPS with the UI running and roughly
180 FPS with it closed. This is not the helper-killing memory optimization the
RAM section warns against, and the Riot client, the League backend and the game
are never touched.

The helper fails closed. It attributes the game and the client to the launcher's
prefix through the loader's __mldr_sockpath, since guest processes do not carry
DPREFIX (checked on a live session). It never closes a client younger than 60
seconds (the one the backend starts after a match), preserves a process of
another installation or owner, acts only when the environment and the stat fields
can be read, gives up after three closes in one match, and leaves after ten
minutes without a client or a match. It is off by default.

Tests use fake /proc entries and mock every signal.

.github/workflows/source-check.yml

@@ -21,3 +21,4 @@ jobs:
       - run: python -m unittest discover -s tests -p 'test_system_profiler.py'
       - 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'