feat: polish CSV header format with improved metadata and column naming

tests/physics/test_multi_branch.py

@@ -464,11 +464,11 @@ class TestMultiBranchCSVExport:
         df = pd.read_csv(csv_path, comment="#")
 
         # Should have total spectrum column
-        assert "spectrum" in df.columns
+        assert "total_spectrum" in df.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
+        assert "branch 1" in df.columns
+        assert "branch 2" in df.columns
 
         # Should NOT have repeated per-branch metadata columns
         assert "branch_1_intensity" not in df.columns
@@ -502,7 +502,7 @@ class TestMultiBranchCSVExport:
 
         df = pd.read_csv(csv_path, comment="#")
 
-        assert "spectrum" in df.columns
+        assert "total_spectrum" in df.columns
         assert "Fermi" in df.columns