Remove 'universal_' prefix from CSV column names in multi-branch mode

Universal components (Fermi, Screening, etc.) are exported under their
plain names — no redundant prefix needed.

beta_spectrum/spectrum.py

@@ -1582,10 +1582,10 @@ class BetaSpectrumAnalyzer:
                 for comp_name, values in branch_comps.items():
                     data[f"branch_{i+1}_{comp_name}"] = values
 
-            # Universal components only (skip per-branch keys)
+            # Universal components (same for all branches — no prefix needed)
             for name, values in components.items():
                 if not name.startswith("branch_"):
-                    data[f"universal_{name}"] = values
+                    data[name] = values
         else:
             # Single-branch mode: standard export
             for name, values in components.items():

tests/test_multi_branch.py

@@ -479,7 +479,7 @@ class TestMultiBranchCSVExport:
         assert "branch_2_transition" not in df.columns
 
         # Should have universal columns
-        assert "universal_Fermi" in df.columns
+        assert "Fermi" in df.columns
 
     def test_single_branch_csv_columns(self, tmp_path):
         """Single-branch CSV should have standard columns."""