Remove repeated per-branch metadata columns from CSV export

Branch intensity, endpoint, and transition are constant per branch and
already documented in the CSV header. No need to repeat them as columns
for every energy row.

tests/test_multi_branch.py

@@ -466,10 +466,18 @@ class TestMultiBranchCSVExport:
         # Should have total spectrum column
         assert "spectrum" in df.columns
 
-        # Should have per-branch columns
+        # Should have per-branch spectrum columns (intensity/endpoint/transition are in the header)
         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
+        assert "branch_1_endpoint" not in df.columns
+        assert "branch_1_transition" not in df.columns
+        assert "branch_2_intensity" not in df.columns
+        assert "branch_2_endpoint" not in df.columns
+        assert "branch_2_transition" not in df.columns
+
         # Should have universal columns
         assert "universal_Fermi" in df.columns