fix(cypher): support multiple reentry WHERE stages - #1032

Refs #1000

Stacked on #1029.

This Phase 4 slice replaces the single global post-reentry WHERE model with aligned per-stage reentry_wheres, so alternating shapes like MATCH ... WITH ... MATCH ... WHERE ... WITH ... MATCH ... WHERE ... WITH ... RETURN compile and execute correctly.

What landed

• parser/AST support for one WHERE per post-WITH MATCH stage
• recursive suffix lowering that preserves and rewrites remaining reentry WHERE clauses stage-by-stage
• amplification for carried scalars, sparse WHERE placement, empty-result behavior, and second-stage fail-fast boundaries
• narrow test-local DRY cleanup plus a CHANGELOG.md note

Local validation

• python3.12 -B -m pytest -q graphistry/tests/compute/gfql/cypher/test_parser.py -k "reentry or unwind" -> 11 passed, 84 deselected
• python3.12 -B -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "cartesian_dynamic_pattern_property or multi_stage_with_match_reentry_with_intermediate_where or multiple_post_with_where_clauses or post_with_match_collect_unwind_match or issue_1000_ic6_after_phase4_now_stops_at_prefix_scalar_with_reentry_requirement" -> 19 passed, 1 skipped, 613 deselected
• ruff check graphistry/compute/gfql/cypher/ast.py graphistry/compute/gfql/cypher/parser.py graphistry/compute/gfql/cypher/lowering.py graphistry/tests/compute/gfql/cypher/test_parser.py graphistry/tests/compute/gfql/cypher/test_lowering.py -> passed

Exact IC6 progression

• previous blocker: Cypher only supports one WHERE clause after post-WITH MATCH
• current blocker after this PR: Cypher MATCH after WITH currently requires the prefix WITH stage to project exactly one whole-row alias

So this PR is a real GREEN slice, but it is not final #1000 closeout. The next stacked slice should tackle scalar-only prefix WITH ... MATCH ... continuation before the comma-pattern fanout lane.