GFQL: scope-aware predicate pushdown barriers (#1190)

[lmeyerov]

Summary

• thread binder scope metadata (BoundIR.scope_stack) through compiled Cypher payloads into runtime PlanContext
• enforce with_barrier_blocks_pushdown() inside PredicatePushdownPass using pass-time scope metadata
• add targeted regressions for WITH-barrier blocked vs allowed pushdown decisions and scope-stack threading in compile route
• add changelog entry for M4-PR5: scope-aware predicate pushdown barriers (WITH/ScopeFrame metadata plumbing) #1190

Implementation details

• PlanContext now carries scope_stack
• CompiledCypherExecutionExtras/CompiledCypherQuery now carry scope_stack
• compile_cypher_query() captures bound scope frames and attaches them to compiled query extras
• gfql_unified logical pass pipeline now builds PlanContext(scope_stack=compiled_query.scope_stack)
• PredicatePushdownPass now checks WITH barriers before pushdown

Tests

• PYTHONPATH=. uv run --no-project --with pytest python -m pytest graphistry/tests/compute/gfql/test_predicate_pushdown_pass.py graphistry/tests/compute/gfql/cypher/test_lowering.py -q
• PYTHONPATH=. uv run --no-project --with pytest python -m pytest graphistry/tests/compute/gfql/test_pushdown_safety.py graphistry/tests/compute/gfql/test_pass_manager.py graphistry/tests/compute/gfql/test_ir_compilation.py graphistry/tests/compute/gfql/test_ir_m0b_conformance.py graphistry/tests/compute/gfql/test_unnest_apply.py -q
• ./bin/ruff.sh
• uvx mypy --cache-dir /tmp/mypy_cache_1190 graphistry/compute/gfql/ir/compilation.py graphistry/compute/gfql/ir/pushdown_safety.py graphistry/compute/gfql/passes/predicate_pushdown.py graphistry/compute/gfql/cypher/lowering.py graphistry/compute/gfql_unified.py

Closes #1190.