- Encode header keys/values as CBOR text strings (major type 3)
- IDCard.to_binary now sets ct=idcard header and uses bottle_up
  before signing, matching Go's IDCard.Sign() behavior

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

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