Adds Key: value header rows under the subject, validated with carrier's
rules: keys of 1-64 letters, digits and dashes, unique case-insensitively,
none of the fields the client sets itself, and the whole block inside the
reader's budget. A reply to a message with the sender's own fields asks
whether to carry copies over.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>