feat: Add the FDv2 transactional store and change-set translation

[beekld]

Summary

FDv2 delivers data as change sets that must be applied to the store as atomic batches. This adds the write path for that:

• A TransactionalDataStore trait, parallel to DataStore, whose apply method applies a change set to the store as a single atomic batch, with an implementation for the in-memory store.
• Translation from wire change sets into typed store change sets, deserializing each object into a Flag or Segment.

Unknown object kinds in a change set are logged and skipped rather than silently ignored, matching the C++ and Java SDKs. The spec calls for silently ignoring them.

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Note

Overview
Adds the FDv2 → store write path: wire change sets are converted into typed ItemChange batches (Flag/Segment puts and tombstone deletes), then applied atomically via a new TransactionalDataStore trait (implemented on InMemoryDataStore).

apply honors None (no-op), Full (clear flags/segments then apply changes), and Partial (merge changes only). Unknown put/delete kinds are warned and skipped; bad flag/segment JSON fails translation with serde_json::Error.

Exposes fdv2::model and shared ChangeSetKind so stores and FDv2 share types; trait/orchestrator wiring is marked for a follow-up phase.

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