Compile fluent chains to Cypher on the Neo4j backends: a chain that would execute N Python-side hops becomes one Cypher round-trip against the graph.

Scope

• Planner decides per stage: pushdown to Cypher vs in-process evaluation.
• Parity tests: same chain, same results, in-process vs Neo4j (extends the contract suite).

This is where the fluent API pays off at very large scale.

Part of #155. Branch: feat/issue-<n>.