fix(output): apply positive-integral export guard to to_one mode

CSVEmitter.export_to_csv raised ValueError only in intensity mode;
degenerate exports with --normalize-to-one silently wrote all-zero
CSV files. The guard now runs before the mode split and no file is
written in either mode.

beta_spectrum/output/csv_export.py

@@ -19,7 +19,7 @@ Column semantics (v0.4.0 onward):
 from __future__ import annotations
 
 from datetime import datetime, timezone
-from typing import TYPE_CHECKING, Dict, Optional
+from typing import TYPE_CHECKING, Dict
 
 import numpy as np
 import pandas as pd
@@ -179,16 +179,17 @@ class CSVEmitter:
             "energy_MeV": analyzer.energies_MeV,
         }
 
-        integral: Optional[float] = None
+        # Positive-integral guard applies to BOTH normalization modes: a
+        # degenerate spectrum must never be written (not even as zeros).
+        total_raw = analyzer.spectrum(analyzer.W)
+        integral = float(np.trapezoid(total_raw, analyzer.energies_MeV))
+        if integral <= 0.0:
+            raise ValueError(
+                "Total spectrum integral is non-positive: every branch "
+                "is degenerate (check neutrino_mass vs branch endpoints "
+                "and intensity_cutoff)"
+            )
         if not to_one:
-            total_raw = analyzer.spectrum(analyzer.W)
-            integral = float(np.trapezoid(total_raw, analyzer.energies_MeV))
-            if integral <= 0.0:
-                raise ValueError(
-                    "Total spectrum integral is non-positive: every branch "
-                    "is degenerate (check neutrino_mass vs branch endpoints "
-                    "and intensity_cutoff)"
-                )
             data["total_spectrum"] = total_raw / integral
 
         for i in range(n_branches):
@@ -205,7 +206,6 @@ class CSVEmitter:
                 else:
                     data[col_name] = np.zeros_like(physical[i])
             else:
-                assert integral is not None
                 intensity_i = analyzer._unified_branches[i].intensity
                 data[col_name] = physical[i] * intensity_i / integral