Previously, SIGTERM immediately re-raised with SIG_DFL, killing
fish without saving history. SIGHUP deferred via a flag but never
re-raised, so the parent saw a normal exit instead of signal death.
Unify both signals: the handler stores the signal number in a single
AtomicI32, the reader loop exits normally, throwing_main() saves
history and re-raises with SIG_DFL so the parent sees WIFSIGNALED.
Fixes#10300Closes#12615