test(regression): set absolute tolerance to 0.5 kmol

- Update regression comparison to use an absolute tolerance of 0.5 kmol
- Replace previous numerical tolerance (1e-8) with a physically meaningful threshold
- Keep relative tolerance (rtol=1e-9) unchanged for floating-point stability

Author: harryswift01

tests/regression/test_regression.py

@@ -151,5 +151,5 @@ def test_regression_matches_baseline(
         got_payload=run.payload,
         baseline_payload=baseline_payload,
         rtol=1e-9,
-        atol=1e-8,
+        atol=0.5,
     )