test(gfql): register the low-cardinality count gate suite in the polars lane

Caught by the repo's own polars-lane completeness meta-test, which is the only
thing standing between a new polars test module and running in no lane at all.
It was the ONLY new failure the full-suite A/B on dgx surfaced (comb vs
comb+gate, RAPIDS image, --gpus all).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YYZRXegrALuXd3NHH5evqx

Author: lmeyerov

bin/test-polars.sh

@@ -38,6 +38,7 @@ POLARS_TEST_FILES=(
     graphistry/tests/compute/gfql/test_seeded_typed_hop_fastpath.py
     graphistry/tests/compute/gfql/test_residual_polars_native.py
     graphistry/tests/compute/gfql/cypher/test_grouped_aggregate_fused_polars.py
+    graphistry/tests/compute/gfql/cypher/test_grouped_aggregate_lowcard_count.py
     # module-level `importorskip("polars")` files that previously ran in no lane at all
     graphistry/tests/compute/gfql/test_engine_polars_narrow_combine.py
     graphistry/tests/compute/gfql/test_engine_polars_semi_key_dedup.py