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.

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()