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.

---

Note

Low Risk
New isolated code with no production integration yet; behavior is confined to unit tests until the data source adopts it.

Overview
Adds a new fdv2 module (registered in lib.rs behind #[allow(dead_code)]) so FDv2 is implemented and unit-tested but not yet hooked into streaming/polling.

wire.rs defines serde models for FDv2 SSE payloads (server-intent, put-object, delete-object, payload-transferred, error, goodbye) plus assembled ChangeSet / FDv2Change types. Flag/segment bodies stay as serde_json::Value; unknown intent codes map to IntentCode::Unknown, and Goodbye / FDv2Error use optional fields for tolerant parsing.

protocol.rs implements FDv2ProtocolHandler, a small state machine that turns those events into ProtocolResult values (noop, ChangeSet, errors, or goodbye). It distinguishes full vs partial transfers, allows further put/delete cycles after the first payload-transferred without a new intent, treats intent none as an empty changeset, clears buffered changes on server error while keeping transfer state, and fully resets on goodbye (with permissive goodbye JSON parsing).

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[keelerm84]

What do we think about using serde's raw value? Might save some cycles if we find we need to discard part of the stream.

[beekld]

I prefer keeping this as Value to be consistent with how it's written in C++ and Java. I'm open to discussing it further.

[cursor]

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