I reclassified the withdrawn `v7.0.0` release as `v6.1.0` while keeping the same renderer behavior and dependency requirements. I retained the documented compatibility changes and marked the historical `v7.0.0` entry withdrawn.

Signed-off-by: Zack Koppert <zkoppert@github.com>
Copilot-Session: 6fe372b1-3ff6-4129-9d33-be564fb6a5f0

* fix: restore legacy CommonMarker compatibility

I retain the existing CommonMarker 2.x adapter and delegate to the original render_html API when the legacy parser is installed. Existing installations keep their parser and its option behavior.

I prepare version `6.1.1` with real-parser regression tests and a legacy full-suite CI job. The incompatible `6.1.0` release has been withdrawn.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 6fe372b1-3ff6-4129-9d33-be564fb6a5f0
Signed-off-by: Zack Koppert <zkoppert@github.com>

* docs: clarify CommonMarker testing and extension behavior

I keep the development version override in the README and place full test commands in CONTRIBUTING. I correct comments about modern-only extensions without changing rendering.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 6fe372b1-3ff6-4129-9d33-be564fb6a5f0
Signed-off-by: Zack Koppert <zkoppert@github.com>

---------

Signed-off-by: Zack Koppert <zkoppert@github.com>
Copilot-Session: 6fe372b1-3ff6-4129-9d33-be564fb6a5f0