- Convert filter operator, error types, mailbox roles, and submission
statuses from regular variants to polymorphic variants
- Add draft-ietf-mailmaint extended keywords: $notify, $muted, $followed,
$memo, $hasmemo, $hasattachment, $hasnoattachment, $autosent,
$unsubscribed, $canunsubscribe, $imported, $istrusted, $maskedemail, $new
- Add Apple Mail flag color keywords ($MailFlagBit0/1/2) with flag_color
type and conversion functions
- Add extended mailbox roles: snoozed, scheduled, memos
- Add RFC 8621 submission-specific errors: forbiddenMailFrom,
forbiddenFrom, forbiddenToSend

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>