feat: Add the FDv1 adapter synchronizer

[beekld]

Summary

Gives the FDv1 fallback an implementation: an adapter that runs an existing FDv1 DataSource and turns its store writes into FDv2 change sets. A full store initialization becomes a full change set and an individual upsert becomes a partial one. A permanent FDv1 initialization failure surfaces to the orchestrator as a terminal error.

The fallback source is built once and re-subscribed on each activation, rather than rebuilt, because the FDv1 data source factory is not Send + Sync and cannot be captured in the synchronizer factory. Re-subscribing is safe, since each subscription starts a fresh stream or poll loop.