GFQL: close #1226 residuals and harden #1219 pushdown guardrails

[lmeyerov]

Summary

• retire graphistry/compute/gfql/expr_split.py and migrate top-level AND splitting into predicate_pushdown.py (split_top_level_and_conjuncts)
• fix parser literal fallback atom text to Cypher keyword casing (true / false / null)
• tighten optional-arm pushdown null-safety by treating OR-compound predicates as conservatively null-rejecting
• update/extend tests for literal fallback atoms, splitter behavior through pushdown path, and optional-arm OR guardrails
• remove obsolete test_expr_split.py and add changelog entry

Issues

• Closes gfql/cypher: post-#1200 follow-ups (literal-atom fidelity, expr_split.py retirement) #1226
• Refs Cypher WHERE: static validation gap on row-boolean shapes (OR / NOT / XOR among row predicates) — emergent from #1217 Earley swap #1219

Validation

• PYTHONPATH=. pytest -q graphistry/tests/compute/gfql/cypher/test_boolean_expr.py graphistry/tests/compute/gfql/cypher/test_where_bool_conformance.py graphistry/tests/compute/gfql/test_predicate_pushdown_pass.py graphistry/tests/compute/gfql/cypher/test_binder_expr_tree.py graphistry/tests/compute/gfql/cypher/test_where_clause_expr_tree_invariant.py
• uv run ruff check graphistry/compute/gfql/cypher/parser.py graphistry/compute/gfql/cypher/_boolean_expr_text.py graphistry/compute/gfql/passes/predicate_pushdown.py graphistry/compute/gfql/ir/pushdown_safety.py graphistry/tests/compute/gfql/cypher/test_boolean_expr.py graphistry/tests/compute/gfql/cypher/test_where_bool_conformance.py graphistry/tests/compute/gfql/test_predicate_pushdown_pass.py
• UV_CACHE_DIR=/tmp/uv-cache uv run mypy graphistry/compute/gfql/cypher/parser.py graphistry/compute/gfql/cypher/_boolean_expr_text.py graphistry/compute/gfql/passes/predicate_pushdown.py graphistry/compute/gfql/ir/pushdown_safety.py
• PYTHONPATH=. pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "issue_1219_row_boolean_audit"