refactor(api): remove the unused interim RecordBatch abstraction

[DecisionNerd]

Split from #1020 after architecture review.

Scope

graphforge-api/src/lib.rs contains a stringly interim RecordBatch abstraction used only by tests while the supported pipeline uses Arrow RecordBatch values.

Acceptance criteria

• Confirm no supported public consumer depends on the interim type
• Delete it or replace remaining test uses with the canonical Arrow/public result contract
• No second record-batch vocabulary remains in the API facade
• API, Python, and Node conformance tests pass