ci(gfql/polars): run cypher test_lowering polars cases in the polars lane

The polars-parametrized cypher tests (numeric fns, toLower, =~ parity in
graphistry/tests/compute/gfql/cypher/test_lowering.py) never executed in CI:
the core lane has no polars installed (importorskip -> skip) and the polars
lane's fixed file list excluded the file. So the polars numeric lowerings this
branch adds were CI-untested and their changed lines uncovered (coverage gate
76.9% < 80). Add the file (-k polars) to bin/test-polars.sh and the ci.yml
polars coverage step (--cov-append). Moved from the stacked #1677 to here — it
belongs with the lowerings it tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lmeyerov

bin/test-polars.sh

@@ -28,3 +28,12 @@ if [ -n "${POLARS_COV:-}" ]; then
 fi
 
 python -B -m pytest -vv "${COV_ARGS[@]}" "${POLARS_TEST_FILES[@]}" "$@"
+
+# cypher-lowering polars-parametrized cases (round ties, lower/upper, =~, numeric fns);
+# appended into the same coverage data file when POLARS_COV=1 (CI audit reads it)
+COV_APPEND_ARGS=()
+if [ -n "${POLARS_COV:-}" ]; then
+    COV_APPEND_ARGS=(--cov=graphistry/compute --cov-report= --cov-append)
+fi
+python -B -m pytest -vv "${COV_APPEND_ARGS[@]}" \
+    graphistry/tests/compute/gfql/cypher/test_lowering.py -k polars