Summary

Child deletion lane from #1468 / PR #1476 residual unplanned-chain fallback audit.

The corpus still has a non_top_level_optional_match fallback bucket. This issue owns native planning for non-top-level OPTIONAL MATCH cases not covered by prior optional work.

Parent / context

• Historical deletion meta: [META] GFQL deletion wave: remove 2K+ LOC of obsolete compatibility paths #1419, closed/de-scoped after GFQL compat-executor reachability shrink #1467.
• Fallback audit: GFQL deletion: retire residual unplanned Cypher chain-execution fallback #1468 / PR Harden residual Cypher unplanned-chain fallback #1476.
• Already landed optional routes: D6 deletion prep: native route for simple top-level OPTIONAL MATCH fallback #1460 top-level optional, D7 deletion prep: native route for OPTIONAL MATCH reentry fallback #1461 optional reentry.
• Current residual bucket: non_top_level_optional_match reported as 10 corpus cases.

Scope

• Inventory the residual non-top-level OPTIONAL MATCH corpus cases.
• Native-plan the cases that fit the existing logical/physical optional machinery.
• Keep unsupported shapes explicitly classified fail-closed.
• Add tests proving covered cases bypass _execute_compiled_query_chain_non_union().
• Update the residual count in closeout.

Out of scope

• Top-level optional already handled by D6 deletion prep: native route for simple top-level OPTIONAL MATCH fallback #1460.
• Optional reentry already handled by D7 deletion prep: native route for OPTIONAL MATCH reentry fallback #1461.
• Multiple MATCH, anonymous MATCH, scalar/projection alias buckets.
• Final fallback allowlist deletion.

Acceptance

• Covered non-top-level OPTIONAL MATCH cases native-plan and bypass residual chain fallback.
• Remaining cases, if any, are enumerated with proposed follow-up split.
• Focused tests cover matched and null-extension behavior where applicable.
• Local focused GFQL tests, ruff/typecheck, PR CI, and DGX RAPIDS 25.02/26.02 GFQL validation are green or explicitly waived by coordinator.