Adds a SAFETY comment explaining why the transmute of sqlite3_vec_init
to the sqlite3_auto_extension callback type is sound. The three
invariants (stable C-ABI signature, single-call-per-connection contract,
idempotency) were previously undocumented, which left the lone unsafe
block without justification for future readers.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>