- Implement Email.t type with proper keyword support
- Create Keywords module with explicit variants for system and custom flags
- Add operations for manipulating email flags
- Add conversion utilities for JMAP/IMAP interoperability
- Update email-related types to use the module style with abstract types
- Convert record types to modules with accessors and constructors

The keyword implementation uses a variant type to make the different
flags more explicit and type-safe, while still supporting custom keywords
via a Custom constructor. Implementation follows RFC 8621 section 4.1.1.

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

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