fix: stabilize with-where scope projection

[lmeyerov]

Summary

• rewrite non-aggregate Cypher WITH ... WHERE filters so active-alias property refs stay vectorized after projection
• lazily add and immediately drop hidden helper columns when a filter needs source properties that were not otherwise projected
• add regression coverage for pre-projection scope, mixed scope predicates, helper-column cleanup, and the explicit WITH DISTINCT failfast boundary

Validation

• full graphistry/tests/compute/gfql/cypher/test_lowering.py passed locally
• py_compile passed on lowering.py and test_lowering.py
• ruff check --no-cache passed on lowering.py and test_lowering.py
• mypy passed on lowering.py
• targeted sibling direct-Cypher probe: with-where7-1/2/3 all pass

Notes

• mypy still reports 4 pre-existing attr-defined errors in graphistry/tests/compute/gfql/cypher/test_lowering.py:181-184 when that test file is included