feat: add server FDv2 data system orchestrator

[beekld]

Add FDv2DataSystem orchestrator

Orchestrates FDv2 initializers and synchronizers; applies changesets and reports data-source status.

What's implemented

• FDv2DataSystem — IDataSystem impl. Runs initializers, then a synchronizer; applies changesets; tracks the selector; emits status (kInitializing → kValid on first apply / kInterrupted on errors → kOff on destruction). Offline (no factories) goes straight to kValid.
• IFDv2InitializerFactory, IFDv2SynchronizerFactory — build a fresh source per call.
• ITransactionalDestination — extends IDestination with Apply(ChangeSet); implemented by MemoryStore and ChangeNotifier.
• ChangeNotifier::Apply — diffs against the current store, updates the dependency tracker, applies to the sink, emits change events. Full diffs per-kind and version-aware; Partial notifies unconditionally per item.

Design decisions

• No shared_ptr<State> for orchestrator-level async safety. Other FDv2 components keep async state in a shared_ptr<State> so callbacks can outlive the public object. This one captures this directly — safe because ~ClientImpl does ioc_.stop() and run_thread_.join() before destroying the data system. The destructor's precondition (caller drains the executor first) is documented on the class.
• New ITransactionalDestination rather than adding Apply to IDestination. Leaves the FDv1 persistent-store path untouched.
• DataSourceStatusManager is non-owning. ClientImpl (shared with FDv1) is its sole owner; the data system only borrows it. Declaration order in ClientImpl guarantees the manager outlives the data system.

Test plan

11 cases in libs/server-sdk/tests/fdv2_data_system_test.cpp cover lifecycle, initializer phase (basis received, basis-skips-remaining, Interrupted advances, ChangeSet without selector continues), and synchronizer phase (apply, Interrupted loops, Goodbye/TerminalError advances, selector forwarding). Mocks resolve futures synchronously; tests run the io_context to drain orchestration before assertions.