feat: add FDv1AdapterSynchronizer wrapping IDataSynchronizer as IFDv2Synchronizer

[beekld]

Summary

Stacked on #539. Adds an adapter that wraps any FDv1 IDataSynchronizer (e.g. the existing StreamingDataSource) and presents it as an IFDv2Synchronizer. FDv1 Init/Upsert callbacks delivered through an internal IDestination are translated into FDv2SourceResult::ChangeSet results (kFull for Init, kPartial for Upsert), with empty selectors and fdv1_fallback=false.

The orchestrator picks this up after SwitchToFDv1Fallback() once a factory whose IsFDv1Fallback() returns true is configured. Wiring the adapter into the public config builders is a separate concern, deferred until the Configuration API work.

Test plan

• 8 unit tests cover: lazy start, no-restart on second Next, Init→kFull, Upsert→kPartial (both overloads), pending-Next-resolved-as-Shutdown-on-close, ShutdownAsync called once on close (and not at all if never started), Next-after-Close
• Full server suite green (486 tests)