GFQL cleanup: retire lowering.py projection-planning compatibility delegates #1514

Refs #1509. Refs #1058.\n\nFollow-up from the #1509 post-reentry lowering.py DRY audit.\n\n## Problem\nA broader projection-planning compatibility delegate family still lives in graphistry.compute.gfql.cypher.lowering even though the implementation owner is graphistry.compute.gfql.cypher.projection_planning. Examples include _split_qualified_name, _projection_ref_from_expr, _reject_duplicate_alias_row_refs, _empty_optional_projection_row, _optional_null_fill_plan, _optional_projection_row_guard_plan, _plan_with_visible_projected_columns, and _projection_output_names.\n\n#1509 intentionally did not remove this family because graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py currently pins some lowering.* delegates for split compatibility, so retiring the family is broader than a narrow dead-code deletion.\n\n## Scope\n- Decide whether the S3 split compatibility delegates are still required.\n- If not required, update/remove the split-guard expectations and call sites deliberately.\n- Keep behavior unchanged; this should be ownership cleanup only.\n- Avoid typed-schema work and new Cypher semantics.\n\n## Acceptance\n- Either retire the delegate family with focused tests updated, or document why the compatibility surface remains required.\n- Focused Cypher lowering tests, ruff, typecheck, and cypher surface guard pass.\n