Companion

Companion object holding the DefaultCollectScope sentinel.

Experimental: the whole companion is @ExperimentalComposePreviewLabApi-gated because everything it currently exposes belongs to the still-stabilizing scope feature. Marking the companion (rather than just the const inside) keeps the Companion class itself out of the BCV baseline — without this marker, the empty companion class signature stays in *.api / *.klib.api baselines forever even after we eventually remove or move DefaultCollectScope, locking in the Companion class as ABI surface even though it has no stable members.