PR #11790 Cursor 5th re-review: stale echo ref blocks re-brush

Cursor Bugbot Medium finding 2026-06-09T02:27:38Z on PR #11790 head 253ee2f5. Location: packages/edges/src/components/charts/ChartBrush/ChartBrush.tsx#L150-L154 + L190-L194. After clearing zoom, lastEchoedDomainRef can still hold the previous brush range. Tier-1 echo guard then treats a new selection with the same timestamps as a duplicate and drops it, so brush zoom may not apply again until the user picks a different window. Fix: clear lastEchoedDomainRef.current = null in the brushRegion useEffect when brushRegion goes null. Add regression test in ChartBrush.test.tsx.