Accept horizontal-tab continuations when unfolding content lines and
allow hyphens in group names, matching the MIME-DIR and vCard grammar.
Also add focused private tests covering both cases.
AI-Assisted: yes
AI-Tool: OpenAI Codex / gpt-5.4 xhigh
- The documentation is cluttered enough as it is with the large number
of procedures supporting vCard 3 and 4. Split common out into the
publicly exposed bits and the private internals. This makes it obvious
which common functionality a client can expect to have exposed on the
main vcard module.
- Add documentation (WIP) on the vcard3 module.
