fix(bwrap): close pdeathsig parent race

[danielchristiancazares]

Summary

• capture parent PID before calling prctl(PR_SET_PDEATHSIG, SIGKILL)
• close the documented race window by re-checking parent PID immediately after prctl
• self-terminate if parent changed before PDEATHSIG could be installed

Testing

• meson test -C _builddir_ubuntu --print-errorlogs
  • 4 OK, 0 FAIL, 1 SKIP

[swick]

This needs a lot more explanation, including what the race actually is, why _exit is used, why it calls kill on itself instead of relying on exit. The PR description also suspiciously looks vibe-coded.