fix: re-export getShadowBackfillStopReason through the embedding barrel

The stop-reason commit imported it in backfill-embeddings.ts through the
barrel but exported it only from project-embedding-registry.ts, leaving
the scripts typecheck leg red (caught by the deletion mason's gate run).

Author: ualtinok

packages/plugin/src/features/magic-context/memory/embedding.ts

@@ -28,6 +28,7 @@ export {
     getPrimaryEmbeddingMeasurementCohort,
     getProjectEmbeddingSnapshot,
     getShadowBackfillRemaining,
+    getShadowBackfillStopReason,
     getShadowEmbeddingMeasurementCohort,
     markProjectLoadUntrusted,
     registerProjectEmbedding,