Issue 12: pass bw unlock password via stdin (--passwordstdin) instead of
as a CLI arg, preventing it from appearing in /proc/<pid>/cmdline or ps output.
Issue 11: replace Nonce::default() (all-zero) with the first 12 bytes of the
16-byte random payload nonce. The key-nonce pair was already unique per message
(HKDF salt), but using an explicit non-zero nonce removes the latent risk of
accidental key reuse and makes the safety invariant self-documenting.
Closes#11, closes#12
