fix(gfql): preserve boolean sum zero on polars gpu

[lmeyerov]

Summary

• Normalize the cuDF-Polars 26.02 all-null Boolean sum result to the GFQL/Cypher zero contract.
• Scope the repair to exactly Boolean sum before the existing Int64 result cast.
• Add direct positive and negative boundary tests.
• Record the fix under [Development] in CHANGELOG.md.

Final linear state

• Head: d3573735040d7f65d00a24f2e08fa5b8a9d83e77
• Base: master at merged fix(gfql): treat binding seed ids as identities #2001 commit d627fac2e7e55168f0ba43b8b1f4e3311b4e2f52
• Topology: five linear, one-parent commits; zero merge commits
• Incremental diff: three files, +24/-0

The four previously reviewed code/test patches are range-diff exact after the final rebase. The fifth commit adds only the requested changelog entry.

Semantics boundary

polars_conform_agg_dtype() fills null only when both conditions hold: the aggregate is sum and the input dtype is Boolean.

The focused test pins:

• Positive: Boolean sum null → Int64 zero.
• Negative function side: Boolean count null → Int64 null.
• Negative dtype side: non-Boolean sum null → unchanged null.

Validation

• Focused CPU boundary selection: 52 passed, 18 expected skips, 425 deselected.
• ./bin/lint.sh: clean, including type-hygiene, comment-density, and import guards.
• ./bin/typecheck.sh: mypy 2.3.1, 336 source files, zero issues.
• No Docker, local GPU, DGX, or broad local suite was used for the final rebase.

Review resolution

• Boundary test request: resolved by the focused positive cell and two negative controls.
• The branch is now directly linear on merged fix(gfql): treat binding seed ids as identities #2001 with no integration merge or predecessor diff leakage.

Landing

Review the master-relative range d627fac2e...d35737350.

After this exact-head CI generation is green, the authorized landing method is GitHub Create a merge commit with admin override. Retain the source branch.

[lmeyerov]

Stacked incorrectly

[lmeyerov]

Resolved on exact head 712e2d121f6441b4cef320469a6c6162ea4a63bd: #2002 is now four one-parent commits directly on exact #2001 head a888c33f2d42f626bb0754fb1186b2d1c1bf6561, with zero integration merges. The incremental diff is exactly two files (+23/-0), and all four patches are range-diff equivalent to their reviewed versions. The PR remains based on the #2001 branch for incremental review; after #2001 lands, the maintainer can retarget #2002 to master, wait for the refreshed exact-head checks, and merge out of band.