refactor(gfql): remove unused cypher lowering delegate - #1516

Summary

• Closes GFQL cleanup: lowering.py post-reentry dead-code and helper ownership audit #1509
• Refs perf: graphistry wheel size (625 kB) — GFQL engine files are large, investigate modularization #1058
• Deletes the unused private lowerer delegate _raise_if_invalid_graph_projection_expr
• Leaves projection validation owned by cypher.projection_planning
• Adds a changelog entry for the post-reentry lowering.py DRY audit
• Filed follow-up GFQL cleanup: retire lowering.py projection-planning compatibility delegates #1514 for the broader projection-planning compatibility delegate family

Validation

python3 -m compileall -q \
  graphistry/compute/gfql/cypher/lowering.py \
  graphistry/compute/gfql/cypher

python3 -m pytest -q \
  graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py \
  graphistry/tests/compute/gfql/cypher/test_lowering.py \
  -k "not cudf"

Result: 954 passed, 12 skipped, 115 deselected.

./bin/ruff.sh \
  graphistry/compute/gfql/cypher/lowering.py \
  graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py

./bin/typecheck.sh
python3 bin/ci_cypher_surface_guard.py
git diff --check

Surface guard: lowering.py lines 8486 <= 9045.

Review Skill

• Converged after 2 local review waves with no findings
• Artifacts: plans/cypher-lowering-dry-audit-1509/review/final-report.md

DGX

Not run. This deletes an uncalled private delegate and does not touch dataframe execution, cuDF/pandas behavior, physical dispatch, or GPU paths.