The HEADER.FIELDS list hand-copied the _AUTOMATED_HEADERS keys, so a key
added there would silently miss the preflight and the sender's full body
would be fetched before being dropped. Build it from the registry (the
IMAP command is byte-identical today). The preflight also repeated the
RFC822 payload unwrap and had three identical warn/consume/continue
branches: one _imap_payload helper now serves both unwraps and the
rejections collapse to one branch (gate reuse + quality Low, convergent).
db45b44ab7