Fix Cypher pattern predicate row expansion - #1451

Summary

• Fixes Direct-Cypher residual: pattern predicate wrong rows under #1369 #1449
• Keeps direct-Cypher WHERE pattern predicates as existence checks instead of rewriting single positives into appended MATCH clauses
• Adds TCK-shaped pandas/cuDF coverage for expr-pattern1-10, expr-pattern1-13, and expr-pattern1-18
• Updates normalizer/lowering tests and changelog

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k 'pattern_predicate or where_pattern or variable_length_where_pattern' -> 38 passed, 3 skipped
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k 'not cudf' -> 934 passed, 12 skipped, 111 deselected
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_ast_normalizer.py graphistry/tests/compute/gfql/cypher/test_parser.py -> 149 passed
• dgx-spark RAPIDS 26.02 focused pattern predicate targets -> 16 passed
• dgx-spark RAPIDS 25.02 focused pattern predicate targets -> 16 passed
• review skill converged after helper-name cleanup; no remaining blocker/important findings