removeVault() overwrote vault.dat with zeros then unlinked it, but never flushed
to stable storage, so the zeros could stay in the OS cache and never reach disk.
flush + fsync before unlink on POSIX (still best-effort on CoW/SSD, but now does
what it claims).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>