Exit status of the launched command is not propagated: gamescope always exits 0 #2540

Current Behavior:

gamescope exits with status 0 whatever the command after -- returns, including when it is
killed by a signal. I use gamescope's headless backend to run automated render checks, and a
failing check looks like a pass to the calling script.

Looking at the source (309eee7), the status seems to be dropped in two places:
gamescopereaper waits for its child with WaitForAllChildren(nPrimaryChild) and then
returns 0 (src/Apps/gamescopereaper.cpp), and the gamescope-wait thread in
steamcompmgr.cpp ignores the status returned by WaitForChild() for the reaper; main()
then falls off the end. Since this is not backend-specific I expect the other backends to
behave the same, but I only tested headless.

Expected: gamescope exits with the primary child's exit status (and 128+N if it was killed
by signal N), like env, timeout or xvfb-run do. If changing the default is a concern, an
option to enable it would also work for me.

Steps To Reproduce:

gamescope --backend headless -- sh -c 'exit 7'; echo $?          # prints 0, expected 7
gamescope --backend headless -- false; echo $?                   # prints 0, expected 1
gamescope --backend headless -- sh -c 'kill -SEGV $$'; echo $?   # prints 0, expected 139

Hardware information:

• Distro: CachyOS (kernel 7.3.1-1-cachyos)
• CPU: AMD Ryzen APU (Renoir)
• GPU: AMD Radeon Graphics (RADV RENOIR)
• Driver Version: Mesa 26.2.6

Software information:

• Desktop environment: KDE Plasma (not involved, headless backend)
• Session type: wayland
• Gamescope version: gamescope version 3.17.0 (gcc 16.2.1), package 3.17.0-1
• Gamescope launch command(s): gamescope --backend headless -- <cmd>

Backends: headless (not in the checkbox list); others not tested.

Checks

• I searched existing issues
• Reproduced with an unmodified gamescope build (distribution package)