fix: use underscores in branch names for CSV headers

tests/physics/test_multi_branch.py

@@ -467,8 +467,8 @@ class TestMultiBranchCSVExport:
         assert "total_spectrum" in df.columns
 
         # Should have per-branch spectrum columns (intensity/endpoint/transition are in the header)
-        assert "branch 1" in df.columns
-        assert "branch 2" in df.columns
+        assert "branch_1_spectrum" in df.columns
+        assert "branch_2_spectrum" in df.columns
 
         # Should NOT have repeated per-branch metadata columns
         assert "branch_1_intensity" not in df.columns
@@ -503,6 +503,7 @@ class TestMultiBranchCSVExport:
         df = pd.read_csv(csv_path, comment="#")
 
         assert "total_spectrum" in df.columns
+        assert "branch_1_spectrum" in df.columns
         assert "Fermi" in df.columns
 
 
@@ -659,9 +660,9 @@ class TestMultiBranchEdgeCases:
         W, _ = spectrum.get_energy_grid(config)
 
         values = spectrum(W)
-        assert np.all(
-            np.isfinite(values)
-        ), "High-Z multi-branch must not produce NaN/inf"
+        assert np.all(np.isfinite(values)), (
+            "High-Z multi-branch must not produce NaN/inf"
+        )
 
     def test_no_branches_defaults_to_single(self):
         """No branches list should default to single-branch mode."""
@@ -726,9 +727,9 @@ class TestMultiBranchEdgeCases:
             contrib = universal_product * branch.intensity * bs[i]
 
             # Branch contribution should not exceed total
-            assert np.all(
-                contrib <= total + 1e-10
-            ), f"Branch {i} contribution exceeds total at some points"
+            assert np.all(contrib <= total + 1e-10), (
+                f"Branch {i} contribution exceeds total at some points"
+            )
 
             # Branch contribution should be non-negative
             assert np.all(contrib >= 0), f"Branch {i} contribution has negative values"
@@ -778,6 +779,6 @@ class TestMultiBranchEdgeCases:
             assert integral > 0, f"Branch {i} has zero integral"
 
         # Sum of branch integrals should equal total
-        assert np.isclose(
-            sum(integrals), total_integral, rtol=1e-6
-        ), "Sum of branch integrals should equal total integral"
+        assert np.isclose(sum(integrals), total_integral, rtol=1e-6), (
+            "Sum of branch integrals should equal total integral"
+        )