Route optional reentry through native planning

[lmeyerov]

Summary

Closes #1461.

• route bounded optional reentry through a narrow logical/physical marker instead of the generic compat executor
• preserve projected scalar carries when optional-reentry null-extension adds unmatched prefix rows
• keep top-level OPTIONAL MATCH deferred for D6 deletion prep: native route for simple top-level OPTIONAL MATCH fallback #1460
• add adversarial runtime/lowering coverage for carried rows, null-extension, matched optional reentry, and unprojected scalar non-leakage

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/test_runtime_physical_cutover.py
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "(1026 or 1047 or 1356) and not cudf"
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "not cudf"
• python3 -m pytest -q graphistry/tests/compute/gfql/test_runtime_physical_cutover.py graphistry/tests/compute/gfql/cypher/test_lowering.py -k "(optional_reentry or issue_1461 or issue_1047 or issue_1356) and not cudf"
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1461_optional_reentry_null_extension_preserves_carried_scalar graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1461_optional_reentry_null_extension_does_not_leak_unprojected_scalar graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1461_optional_reentry_mixed_carried_rows_preserve_null_extended_prefixes graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1461_optional_reentry_matched_single_prefix_remains_native_semantics graphistry/tests/compute/gfql/test_runtime_physical_cutover.py::test_optional_reentry_route_bypasses_compat_executor_and_null_fills
• ./bin/ruff.sh graphistry/compute/gfql/cypher/reentry/compiletime.py graphistry/compute/gfql/cypher/reentry/execution.py graphistry/compute/gfql_unified.py graphistry/tests/compute/gfql/test_runtime_physical_cutover.py graphistry/tests/compute/gfql/cypher/test_lowering.py
• ./bin/typecheck.sh
• git diff --check
• DGX RAPIDS 26.02 targeted matrix: cudf 26.02.01, cuml 26.02.00, 24 passed
• DGX RAPIDS 25.02 targeted matrix: cudf 25.02.02, cuml 25.02.01, 24 passed with known ptxcompiler warning

Review

• Review skill converged after 3 waves. Wave 1 found and fixed one projection-leak risk; waves 2 and 3 were clean.