[codex] Tie #1415 to IC6 tag-cooccurrence coverage

[lmeyerov]

Refs #1415.

Summary

• Tie the existing IC6 tag-cooccurrence join+aggregation regression coverage to post-rerun residual issue [POST-RERUN] #880 residual: tag-cooccurrence join-aggregation bucket #1415 while preserving the original [FEA] GFQL residual row-bindings: tag cooccurrence join aggregation #1396 lineage.
• Update the unreleased changelog entry to cite [POST-RERUN] #880 residual: tag-cooccurrence join-aggregation bucket #1415 for the tag_cooccurrence_join_aggregation [FEA] GFQL bindings_table (row bindings) from chain using Yannakakis prune #880 residual lane.

Context

• [FEA] GFQL residual row-bindings: tag cooccurrence join aggregation #1396 was closed by GFQL: tag-cooccurrence join aggregation cardinality hardening (#1396) #1400 after adding the substantive IC6 regression and cuDF parity guard.
• [POST-RERUN] #880 residual: tag-cooccurrence join-aggregation bucket #1415 is the post-rerun residual follow-up for the same interactive-complex-6 / tag-cooccurrence bucket.
• This PR intentionally does not claim closure until fresh rerun evidence confirms the lane no longer reports backend=gfql, status=partial, issue_ref=graphistry/pygraphistry#880.

Landing Gates

• Blocked until [POST-RERUN] #880 residual: joined row projection bucket #1411 and [POST-RERUN] #880 residual: joined row aggregation bucket #1413 are closed and this branch has been rebased on the PRs that closed them.
• [POST-RERUN] #880 residual: joined row projection bucket #1411 is closed by Add GFQL row join apply (#1411) #1426 and this branch is based on origin/master at 8e7d66c8a57ab6260daf61ef3897375106d78725, the Add GFQL row join apply (#1411) #1426 merge commit.
• [POST-RERUN] #880 residual: joined row aggregation bucket #1413 is still open, linked to [WIP] Fix #1413 joined-row aggregation residual #1422, so this PR must remain draft/blocked.
• DGX RAPIDS 25.02 + 26.02 validation passed on dgx-spark.

Validation

• ./bin/lint.sh CHANGELOG.md graphistry/tests/compute/gfql/cypher/test_lowering.py
• python3 -m pip install --target /tmp/pyg1415-deps 'lark>=1.1,<2'
• PYTHONPATH=/tmp/pyg1415-deps python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1396_issue_1415_tag_cooccurrence_join_aggregation_counts graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1000_ic6_compiles_after_phase6_connected_star_fanout graphistry/tests/compute/gfql/cypher/test_lowering.py::test_string_cypher_executes_issue_1000_ic6_exact_runtime_minimal graphistry/tests/compute/gfql/cypher/test_lowering.py::test_string_cypher_executes_scalar_prefix_reentry_connected_star_comma_fanout_grouped_count -> 4 passed
• PYTHONPATH=/tmp/pyg1415-deps python3 -m pytest -q graphistry/tests/benchmarks/gfql/test_benchmark_residual_triage.py::test_bucket_classification -> 1 passed
• git diff --check
• ssh dgx-spark 'cd /tmp/pygraphistry-issue-1415 && MATRIX_CELLS="25.02:gfql 26.02:gfql" WITH_IMAGE_BUILD=0 WITH_GPU=1 TEST_FILES="graphistry/tests/compute/gfql/cypher/test_lowering.py::test_issue_1396_issue_1415_tag_cooccurrence_join_aggregation_counts_on_cudf" ./docker/test-rapids-official-matrix.sh' -> 25.02/gfql PASS, 26.02/gfql PASS

Local Caveats

• System python3 was missing lark; direct install is blocked by PEP 668, so validation used a /tmp target install.
• Local cuDF tests cannot run because this host has cuDF importable but no CUDA-capable device (cudaErrorNoDevice).
• Fresh rerun evidence is still required before [POST-RERUN] #880 residual: tag-cooccurrence join-aggregation bucket #1415 should close.