The first Xabort fix used a process-directed kill(2), which delivers SIGABRT
asynchronously; the calling thread could race ahead to the fallback panic and
exit(2) before the signal landed — observed as a rare SQLite crash-test failure
(writecrash-1.95.1) after ~94 crash iterations had already passed.

Deliver SIGABRT synchronously and thread-directed via _lwp_kill(_lwp_self())
(the primitive the Go runtime's raise() uses), after unblocking SIGABRT on the
calling thread (__sigprocmask14 SIG_UNBLOCK) so a blocked mask cannot defeat it.
The process-directed kill(2) and panic remain as unreachable fallbacks.

Validated on NetBSD 10.1/amd64: 20/20 child aborts die by signal SIGABRT, both
on the happy path and with SIGABRT pre-blocked on the calling thread.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>