fix(cypher): support scalar-only prefix MATCH reentry - #1034

Refs #1000

Stacked on #1032.

This Phase 5 slice removes the old bounded-reentry requirement that the prefix WITH stage project exactly one whole-row alias. Single-row scalar-only prefix shapes like MATCH (t) WITH t.id AS tid MATCH (p)-[:R]->(u {id: tid}) RETURN ... now compile and execute in the local Cypher compiler.

What's included:

• scalar-only prefix WITH ... MATCH ... support for single-row prefix shapes
• test amplification for multi-scalar carry, empty-prefix behavior, and multi-row fail-fast
• fail-fast locking for unsupported scalar-prefix ORDER BY / LIMIT
• small test-local DRY helper for the repeated prefix-reentry query skeleton
• CHANGELOG.md prep

Benchmark progression:

• exact IC6 now advances past the prefix scalar-carrier blocker
• next blocker is the reentry comma-pattern fanout around post

Focused validation run locally on April 2, 2026:

• python3.12 -B -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "scalar_only_prefix_with_match_reentry or issue_1000_ic6_after_phase5_now_stops_at_reentry_comma_pattern_fanout" -> 8 passed, 1 skipped, 632 deselected
• 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 "multiple_post_with_where_clauses or scalar_only_prefix_with_match_reentry or post_with_match_collect_unwind_match or issue_1000_ic6_after_phase5_now_stops_at_reentry_comma_pattern_fanout" -> 17 passed, 2 skipped, 622 deselected
• ruff check graphistry/compute/gfql/cypher/lowering.py graphistry/compute/gfql_unified.py graphistry/tests/compute/gfql/cypher/test_lowering.py graphistry/tests/compute/gfql/cypher/test_parser.py -> passed