* feat(history): version rows carry their writer, line counts and create marker; a diff endpoint pairs each row with its own change (PLAN-2348 U2)
* test(history): server legs, export/import leg; web type and API method for the version diff (PLAN-2348 U2)
* docs(history): the version diff endpoint and row fields in CLAUDE.md; FormatDiffSummary names what it counts (PLAN-2348 U2)
* fix(history): user_id already existed on item_versions (migration 012); a diff refuses a body the chain could not rebuild (PLAN-2348 U2, codex r1)
xarmian