One Arc per authored sublayer of a layer — the SUBLAYER family (A3-followup), the LAYER-entity analogue of BuildUsdArcs for REFERENCE/PAYLOAD. Each arc carries SUBLAYER.ASSET (the authored path), SUBLAYER.IS_MISSING (does it resolve relative to the owning layer — the same test as REFERENCE.IS_MISSING), and SUBLAYER.LAYER_OFFSET. Existential / correlated matching falls out of the shared FamilyMatch
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