I suggest to split the task of implementing the new extends="…" attribute into smaller technical tickets, starting with this one here:
- In the text, a <ref> with the new attribute is rendered with a sub-number like "1.3".
- In the <references /> section, the reference is rendered with indentation and sub-numbering.
- Include parser tests if applicable.
- Don't deal with any edge cases, where possible. STATUS: accidentally dealt with almost every edge case.
- Don't touch VisualEditor use cases.
Depends on: T236257
This is the recent proof-of-concept, which visually accomplishes the task here: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Cite/+/530399/