Cover the full probe staleness detection path that was previously
untested: stale probe forces reconnect, data changes prevent false
staleness, no-probe config skips staleness check, probe tag subscribed
on connect and protected from unsubscribe.
5 new tests, 184 total passing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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