Replace the tuple(list[dict], int) return of
Session.retain_recent_legal_suffix with a named RetentionResult
dataclass that exposes retained, dropped,
already_consolidated_count, and new_last_consolidated fields.
The tuple return was easy to misuse because the second value only
made sense relative to the first and the old last_consolidated
cursor. The named fields make the archive-skip semantics explicit
at every call site.
No behavior change. All existing tests pass unchanged in semantics.
Refs #4136
Signed-off-by: axelray-dev <110029405+axelray-dev@users.noreply.github.com>