The gauge test polls baseline+1, but the process-global gauge's baseline is
captured once: a parallel class's streams closing after capture lowers the
gauge below baseline+1 forever. Survey confirmed only SiteStreamGrpcServerTests
and CleanupVerificationTests drive the real SubscribeInstance handler that
moves the counter; both now share the SiteStreamSubscriberGauge collection
(row-96 pattern). 4/4 full-suite runs green, durations unchanged.