Evaluate OPTIONAL MATCH last, as a post-join pass after all required MATCHes

[matiastoro]

Motivation

Reorder query evaluation so all required MATCHes join first (maximally constraining the binding set), then apply OPTIONAL MATCH clauses last as left-outer extensions. Evaluating optionals against the smallest possible intermediate reduces work.

Current state (feasibility)

• run_match_chain (src/runtime/engine.rs) threads the environment through the clause chain in source order. Each OPTIONAL MATCH is handled where it appears via either optional_via_bind_pushdown (correlated nested-loop, pinning shared Node vars) or left_outer_join per clause.
• The typechecker models the left-join with outer_join (TLEFTJOIN, src/typing/type_environment.rs).
• Query::has_any_optional() already distinguishes optional clauses.

Proposal

Split the chain into (required[], optional[]); evaluate the required join fully, then fold each optional as a final left-outer extension over the result.

Feasibility

Doable and mostly a scheduling change. Constraints to preserve:

• ISO semantics: an optional that matches nothing must still yield the left row with nulls for its variables.
• Correlation: an optional referencing a required variable still resolves it (already the case with bind-pushdown).
• Variable scoping across the reorder must be unchanged.

Moderate effort. Should be measured against the current in-order pushdown (which already wins on IS5/IC4/IC10-style shapes) to confirm the reorder does not regress correlated cases.