checkRendererSkew deferred the warning whenever the marker file existed;
Desktop no longer deletes dead markers (A7 rule 3), so a crashed update's
leftover suppressed it indefinitely. The skew checker now accepts an async
isUpdating and main defers only on a LIVE marker (readLiveUpdateMarker,
owner liveness), the in-flight update, or the hand-off. A sync predicate
keeps the check synchronous (review K132345 P3 renderer-skew-dead-marker).
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