Summary

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

The corpus still has a single scalar_projection_alias_match fallback case. This should be a small deletion-enabling lane for the scalar/projection alias planner classification gap.

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.
• Current residual bucket: scalar_projection_alias_match reported as 1 corpus case.

Scope

• Reproduce the scalar/projection alias MATCH case that still defers to chain execution.
• Fix planner classification and native route for that shape.
• Add tests proving the case bypasses _execute_compiled_query_chain_non_union().
• Remove or shrink the scalar_projection_alias_match approved fallback bucket if it reaches zero.

Out of scope

• Multiple MATCH fallback bucket.
• Non-top-level OPTIONAL MATCH fallback bucket.
• Anonymous MATCH fallback bucket.
• Final global fallback deletion unless all buckets are empty.

Acceptance

• The scalar/projection alias residual case native-plans.
• The scalar_projection_alias_match residual count is zero, or any remaining case is explicitly documented.
• Focused regression test asserts no residual chain fallback for the covered query.
• Local focused GFQL tests, ruff/typecheck, PR CI, and DGX RAPIDS 25.02/26.02 GFQL validation are green or explicitly waived by coordinator.