feat: add FDv1AdapterSynchronizer wrapping IDataSynchronizer as IFDv2Synchronizer

[beekld]

Summary

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)

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Note

Medium Risk
Touches core flag data sync and fallback behavior with async lifecycle and threading; risk is mitigated by isolation behind IFDv2Synchronizer and broad unit tests, but mis-mapping status or shutdown could affect live updates until config is wired.

Overview
Adds FDv1AdapterSynchronizer, which wraps an FDv1 IDataSynchronizer (e.g. streaming) and exposes it as IFDv2Synchronizer for FDv2 orchestration and FDv1 fallback paths.

An internal ConvertingDestination turns FDv1 Init/Upsert into FDv2SourceResult::ChangeSet (kFull / kPartial, empty selectors, fdv1_fallback=false). Next() lazily starts the wrapped source once, queues multiple results in FIFO order, and races close vs. pending work via WhenAny. DataSourceStatusManager errors map to Interrupted or TerminalError.

The new sources are registered in CMakeLists.txt. 12 gtest cases cover lifecycle, conversion, queuing, status mapping, and shutdown. Public config wiring is left for a follow-up.

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