GFQL compat-executor reachability shrink

[lmeyerov]

Summary

Fixes #1466.

• audited _execute_compiled_query_compat_non_union() reachability after the native physical-dispatch lanes landed
• planned projection-level WITH/RETURN DISTINCT through the logical route so the observed residual defer no longer needs a compat executor
• removed the stale _execute_compiled_query_compat_non_union() wrapper and routed residual logical_plan is None feature-gap guards directly through the canonical chain executor
• updated runtime cutover tests to assert physical planner route use without depending on the deleted private wrapper symbol

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/test_logical_planner.py graphistry/tests/compute/gfql/cypher/test_lowering.py::test_compiled_query_sets_logical_plan_route_for_distinct_projection_shape graphistry/tests/compute/gfql/test_runtime_physical_cutover.py
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py graphistry/tests/compute/gfql/test_runtime_physical_cutover.py graphistry/tests/compute/gfql/test_logical_planner.py graphistry/tests/compute/gfql/test_physical_planner.py -k 'not cudf'
• ./bin/ruff.sh graphistry/compute/gfql/logical_planner.py graphistry/compute/gfql_unified.py graphistry/tests/compute/gfql/test_logical_planner.py graphistry/tests/compute/gfql/cypher/test_lowering.py graphistry/tests/compute/gfql/test_runtime_physical_cutover.py
• ./bin/typecheck.sh
• git diff --check
• DGX RAPIDS 25.02 GFQL/cuDF: 2377 passed, 22 skipped, 15 xfailed
• DGX RAPIDS 26.02 GFQL/cuDF: 2377 passed, 22 skipped, 15 xfailed
• review skill: converged after 2 clean waves; credentials gate clean
• GitHub CI: green, including both tck-gfql runs