coverage: enable parallel data files so modular coverage aggregates across files (#574)

The modular coverage check runs each file's tests in its own `coverage run`
subprocess and merges the results with `coverage combine`. Without parallel data
files every subprocess writes the same `.coverage` and overwrites the previous
one, so `combine` finds nothing to merge and the final report reflects only the
subprocess that happened to finish last. Whenever that last file is fully
covered the check reports success even if other files have gaps. Setting
`run.parallel = true` gives each subprocess a uniquely suffixed data file, so
`combine` aggregates them all and the report spans every file.

Correct aggregation surfaced two spots that no test exercised directly and that
the missing aggregation had hidden: the `QuantumReedMullerCode.order`/`size`
getters and the `TrivialGroup.to_ring_array` defunct-alias error. Add direct
assertions for both.

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

Author: perlinm

pyproject.toml

@@ -104,6 +104,7 @@ ini_options.filterwarnings = [
 
 [tool.coverage]
 run.include = [ "./*" ]
+run.parallel = true
 report.fail_under = 100
 report.exclude_lines = [ "if TYPE_CHECKING:", "pragma: no cover" ]
 report.skip_covered = true

src/qldpc/abstract/groups_test.py

@@ -93,6 +93,10 @@ def test_trivial_group() -> None:
     assert group.equiv(abstract.Group.from_generating_mats())
     assert str(group) == "TrivialGroup"
 
+    # to_ring_array is a defunct alias that only raises, directing callers to RingArray.build
+    with pytest.raises(ValueError, match="DEFUNCT"):
+        abstract.TrivialGroup.to_ring_array([[1]])
+
 
 def test_group_equality_and_equivalence() -> None:
     """``==`` requires a matching representation; ``equiv`` compares underlying groups only."""

src/qldpc/codes/quantum_test.py

@@ -1017,6 +1017,7 @@ def test_reed_muller_css_codes() -> None:
         ((2, 10), (1024, 912, 8)),
     ]:
         code = codes.QuantumReedMullerCode(order, size)
+        assert (code.order, code.size) == (order, size)
         assert code.get_code_params() == params
         assert code.get_distance() == params[2]
         assert code.get_distance(Pauli.X) == params[2]