feat: Add FDv2 wire format and protocol handler

[beekld]

Summary

• Introduces the fdv2 module:
  • wire-format types
  • assembled ChangeSet / Selector types
  • protocol-handler state machine that folds SSE events into change sets
• Object bodies stay as raw serde_json::Value; typed deserialization will live in a later translator.
• Matches the C++ SDK on permissive-parsing choices:
  • Selector keeps the deprecated version field alongside state
  • Goodbye and FDv2Error use optional fields to tolerate permissively-parsed wire data
• Module is annotated #[allow(dead_code)] for now: types are covered by unit tests but not yet reachable from the production data path.