GFQL: tag-cooccurrence join aggregation cardinality hardening (#1396) - #1400

Summary

• add focused [FEA] GFQL residual row-bindings: tag cooccurrence join aggregation #1396 regression fixture for IC6 tag-cooccurrence join+aggregation
• assert non-trivial grouped counts on the exact query shape (collect(distinct friend) -> UNWIND -> connected comma MATCH -> WITH tag.name, count(post))
• add cuDF parity guard for the same lane
• record the lane hardening in CHANGELOG.md

Validation

• uv run ruff check graphistry/tests/compute/gfql/cypher/test_lowering.py
• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "issue_1396_tag_cooccurrence or issue_1000_ic6 or connected_star_comma_fanout_grouped_count"

Closes #1396