* fix(session): repair interrupted turns during V3 migration
* docs(session-format-v3-to-v4): state reference remapping in the summary and code
The frontmatter summary still promised preserved references after the edge
began remapping audited same-artifact references around an inserted
interrupted turn/end. Two local comments record why the opaque branch moves
only `seq` and why the open-step flag needs no reset at a turn boundary.
* docs(session-format-catalog): describe the V3-to-V4 edge as a converting link
The catalog listed the edge as an identity conversion, which stopped being
true once it inserted an evidenced interrupted turn/end and remapped the
audited local references that follow.