ci(packaging): classify kill-after probe timeouts and keep output tails

Address review feedback on the time-boxed runtime probes:

- GNU timeout exits 137, not 124, when -k escalates to SIGKILL. Run it
  with --verbose and report a timeout for 124, or for 137 only when
  timeout announced its own KILL, so an app killed by something else is
  not misreported. Verified against coreutils 9.4 on ubuntu:24.04.
- Print the first and last 16 KiB of oversized Flatpak probe output so
  a late startup exception or the timeout notice stays in the log.
- Pin the hostile Snap probe's timeout-before-generic-failure ordering.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/development/electron-debugging.md

@@ -92,11 +92,12 @@ 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
+`build-and-make.yaml` run under GNU `timeout --verbose -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.
+five minutes with a distinct `::error::` (exit 124, or 137 after timeout's
+announced KILL escalation) and its stderr in the step log, instead of holding
+the job until the 120-minute limit.
 
 ## Main-process ownership