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
@@ -179,8 +179,8 @@ class CSVEmitter:
             "energy_MeV": analyzer.energies_MeV,
         }
 
-        integral: Optional[float] = None
-        if not to_one:
+        # 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:
@@ -189,6 +189,7 @@ class CSVEmitter:
                 "is degenerate (check neutrino_mass vs branch endpoints "
                 "and intensity_cutoff)"
             )
+        if not to_one:
             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
 

tests/physics/test_core_spectrum.py

@@ -66,7 +66,7 @@ def make_single_config(**over: Any) -> SpectrumConfig:
     )
 
 
-def make_degenerate_config() -> SpectrumConfig:
+def make_degenerate_config(**over: Any) -> SpectrumConfig:
     """Multi-branch config where every endpoint lies below m_nu."""
     return SpectrumConfig(
         Z_parent=43,
@@ -75,6 +75,7 @@ def make_degenerate_config() -> SpectrumConfig:
         endpoint_MeV=0.294,
         e_step_MeV=0.001,
         neutrino_mass_MeV=0.35,
+        **over,
         branches=[
             BranchConfig(endpoint_MeV=0.294, transition_type="F2", intensity=0.98),
             BranchConfig(endpoint_MeV=0.200, transition_type="F2U", intensity=0.02),
@@ -329,8 +330,22 @@ class TestCSVExportCoreSemantics:
         spectrum = BetaSpectrum.from_config(config)
         analyzer = BetaSpectrumAnalyzer(spectrum, config)
         path = str(tmp_path / "degenerate.csv")
-        with pytest.raises(ValueError):
+        with pytest.raises(ValueError, match="non-positive"):
             analyzer.export_to_csv(path)
+        assert not (tmp_path / "degenerate.csv").exists()
+
+    def test_degenerate_export_raises_in_normalize_to_one_mode(
+        self, tmp_path: Any
+    ) -> None:
+        """The positive-integral guard must apply in to_one mode too:
+        no all-zero CSV may be written for a degenerate spectrum."""
+        config = make_degenerate_config(normalize_to_one=True)
+        spectrum = BetaSpectrum.from_config(config)
+        analyzer = BetaSpectrumAnalyzer(spectrum, config)
+        path = str(tmp_path / "degenerate_to_one.csv")
+        with pytest.raises(ValueError, match="non-positive"):
+            analyzer.export_to_csv(path)
+        assert not (tmp_path / "degenerate_to_one.csv").exists()
 
     def test_concise_values_match_full(self, tmp_path: Any) -> None:
         config = make_multi_config()