fix(gfql): reject scalar string integer subscripts - #1452

Summary

• Fix direct-Cypher scalar string integer subscript drift for Direct-Cypher residual: string list-index expected-error drift under #1369 #1450 by rejecting scalar string bases before Python string indexing fallback
• Add positive list-index and negative string-index Cypher regressions
• Update CHANGELOG.md under Development / Fixed
• Sidecar tck-gfql rebaseline: Rebaseline string index expected-error drift tck-gfql#116

Fixes #1450

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py::test_string_cypher_supports_list_subscript_with_integer_index graphistry/tests/compute/gfql/cypher/test_lowering.py::test_string_cypher_rejects_string_subscript_with_integer_index graphistry/tests/compute/gfql/cypher/test_lowering.py::test_string_cypher_supports_dynamic_map_subscripts
• python3 -m pytest -q graphistry/tests/compute/gfql/test_row_pipeline_ops.py -k "dynamic_subscript and not cudf"
• PYTHONPATH=/home/lmeyerov/Work/pygraphistry3:/tmp/tck-gfql-issue-1369-fresh /tmp/tck-gfql-ci-venv312/bin/python -m tests.cypher_tck.sweep_direct_cypher --show-nonvalidation-debt --nonvalidation-limit 20 (expr-list1-6-4 now passes expected oracle)
• ./bin/ruff.sh graphistry/compute/gfql/row/pipeline.py graphistry/tests/compute/gfql/cypher/test_lowering.py
• git diff --check
• tck-gfql sidecar focused: 13 passed
• tck-gfql sidecar full: 3935 passed, 763 xfailed
• dgx-spark RAPIDS 25.02 targeted GFQL/cuDF: 3 passed
• dgx-spark RAPIDS 26.02 targeted GFQL/cuDF: 3 passed

Review

• review skill converged after 2 waves with 0 blocker / 0 important / 0 suggestions
• artifacts: plans/issue-1450-string-indexing/review/final-report.md (local only)

Notes

• Local broad sentinel is not a clean gate on this host: repo-root run collected ignored plan artifacts, and scoped run hit unrelated local no-GPU/cuDF/environment failures. Required cuDF paths were validated on dgx-spark.