fix(cypher): support joined-row aggregate overlap - #1036

Refs #1000

Stacked on #1035.

This follow-on slice broadens the joined bindings-row lowering so connected multi-pattern MATCH clauses with aggregate stages also use that row-join path, even when the pattern remains linear and endpoint-stitchable.

What's included:

• grouped overlap support for connected multi-pattern fanout (RETURN c.id, count(d))
• count(DISTINCT d) parity on the same joined-row path
• reentry + grouped aggregate overlap coverage (WITH b, b.id AS bid MATCH ... RETURN bid, count(d))
• cuDF regression coverage for the grouped overlap lane
• small changelog wording update to reflect the broadened joined-row scope

Why this is separate from #1035:

• fix(cypher): support connected comma fanout #1035 fixes the non-linear connected comma-fanout blocker that was directly stopping IC6
• this PR picks up the adjacent linear grouped-overlap limitation without conflating the two review surfaces

Current stack note:

• exact official IC6 now continues past this slice and is fixed on stacked PR fix(cypher): run exact ic6 tag cooccurrence #1037
• keep this PR focused on the grouped-overlap/runtime-join broadening only

Focused validation run locally on April 2, 2026:

• python3.12 -B -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "connected_multi_pattern_grouped_aggregate_overlap or connected_multi_pattern_grouped_distinct_aggregate_overlap or with_match_reentry_connected_multi_pattern_grouped_aggregate_overlap or connected_star_comma_fanout or issue_1000_ic6_compiles_after_phase6_connected_star_fanout" -> 7 passed, 2 skipped, 640 deselected
• python3.12 -B -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "connected_multi_pattern or connected_star_comma_fanout or scalar_only_prefix_with_match_reentry or multiple_post_with_where_clauses or post_with_match_collect_unwind_match or issue_1000_ic6_compiles_after_phase6_connected_star_fanout" -> 32 passed, 5 skipped, 612 deselected
• python3.12 -B -m pytest -q graphistry/tests/compute/gfql/cypher/test_parser.py -k "reentry or unwind" -> 11 passed, 84 deselected
• ruff check graphistry/compute/gfql/cypher/lowering.py graphistry/tests/compute/gfql/cypher/test_lowering.py graphistry/tests/compute/gfql/cypher/test_parser.py -> passed