ci(packaging): time-box the Snap and Flatpak embedded MPV runtime probes

Wrap the three packaged `--embedded-mpv-runtime-probe` launches in GNU
`timeout -k 10 300`, report exit 124 as a distinct `::error::` that says
the packaged app never exited, capture and print the hostile-environment
Snap probe output like the other two probes, and set
ELECTRON_ENABLE_LOGGING=1 so main-process exceptions reach the step log.
A main process that throws before app ready (seen in #1696 with "Cannot
find module './main.app.js'") blocks on Electron's uncaught-exception
dialog under xvfb and previously held the job until its 120-minute
limit with no useful output.

The probe's JSON verdict is still one stdout line, so the exact
`grep -Fx` match on the disconnected Snap probe is unchanged; verified
locally with the workspace Electron binary.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/development/electron-debugging.md

@@ -91,6 +91,12 @@ exit. The retained handle still provides the actual exit code and signal.
 The Linux portable build uploads `packaged-frame-copy-smoke` reports and traces
 even when the smoke fails. Check the paused-frame screenshot and trace before
 classifying a zero rendered-frame signal as an infrastructure flake.
+The Snap and Flatpak `--embedded-mpv-runtime-probe` launches in
+`build-and-make.yaml` run under GNU `timeout -k 10 300` with
+`ELECTRON_ENABLE_LOGGING=1`, so a main process that throws before app ready
+and blocks on Electron's uncaught-exception dialog under xvfb fails within
+five minutes with a distinct `::error::` for exit 124 and its stderr in the
+step log, instead of holding the job until the 120-minute limit.
 
 ## Main-process ownership