currentSid is snapshotted before the awaited /api/session fetch; if the user
clicks a healthy session while a boot-time restore is in flight and that boot
load then fails non-404, _clearStuckSessionOnBoot(sid, null) would wipe the
healthy session's localStorage/URL. Guard the catch block on
_loadingSessionId !== sid (a newer load superseded this one) — re-arm the active
stream and bail before any self-heal/DOM mutation. Protects both the non-404 and
404 inline self-heal paths. + regression test.