Salvaged from PR #62805 (@floatingrain), whose diagnosis of the deterministic
frontend self-abort (createBackendSessionForSend mutating the selected ref +
route, then the caller's drift guard reading its own re-home as a user switch)
was correct — and correct about the sweeper misread that closed it: the
sweeper's implemented_on_main verdict cited submit.ts:245, which was inside
the session.resume block, while the raw post-create guard at the
createBackendSessionForSend site was still aborting every new chat on the
then-current main (4281151ae8). The mechanism itself has since landed via
8c288760d0 + 1bdd478efa, so what remains distinct is this regression: after
the pipeline adopts the created chat as its pinned target, a GENUINE user
switch (selection and route both moving to another chat during the
attachment-sync await) must still abort instead of being masked by the
re-baseline.
Part of the #63078 fix branch.
