#3313 applies the ephemeral-field carry-forward at all 5 wholesale S.messages
replace sites (loadSession/_ensureMessagesLoaded/_loadOlderMessages/
_ensureAllMessagesLoaded/startGatewaySSE), renaming the replace RHS from
msgs/next to _msgsToAssign/_nextToAssign. These pre-existing tests pinned the
old literals; updated to match the new RHS (or made LHS-agnostic) while
preserving the invariants they protect (bump-before-replace, session-switch
guard, shorter-transcript guard).
nesquena-hermes