docs(architecture): scope Arrow result contract to data-returning operations

[DecisionNerd]

Summary

• Replace the universal Arrow-return claim in architecture overview with a data-plane vs control/construction-plane contract, including an inventory of intentional non-Arrow returns (labels, node_count, explain, lifecycle (), construction handles).
• Reconcile API reference, guides, contributor/engineering docs, and AGENTS.md so they no longer imply every method returns Arrow or that bindings own tabular reconstruction.
• Keep Cypher and analyst/data-bearing Arrow guarantees strong; no runtime API changes.

Test plan

• Search docs for contradictory universal claims (all methods return Arrow, bespoke result type, Unified result contract) — clean
• Cross-check overview inventory against Rust (labels/node_count/explain/add_node) and Python/Node projections
• Docs CI / documentation checks on PR head

Closes #708

Made with Cursor

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[coderabbitai]

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