Add massive neutrino support and improve CSV/CLI output

- Add neutrino mass parameter to SpectrumConfig and CLI interface
- Update PhaseSpace component to handle neutrino mass effects properly
- Update CSV headers to show basic endpoint and all transition types
- Add --csv-verbosity option to CLI with 'full' and 'concise' options
- Fix comparison plots to properly show linear scale variations
- Add comprehensive tests for neutrino mass functionality

tests/physics/test_neutrino_mass_effects.py

@@ -0,0 +1,127 @@
+"""
+Physics tests for neutrino mass effects on beta spectrum.
+
+Following TDD principles: write failing tests first, then ensure implementation fixes them.
+"""
+
+import numpy as np
+
+from beta_spectrum.components.phase_space import PhaseSpace
+from beta_spectrum.spectrum import SpectrumConfig, BetaSpectrum
+
+
+class TestNeutrinoMassPhaseSpaceEffects:
+    """Test neutrino mass effects on phase space calculation."""
+
+    def test_phase_space_with_zero_neutrino_mass(self):
+        """Test that phase space calculation works with zero neutrino mass."""
+        ps = PhaseSpace(W0=5.0, transition_type="A", m_nu=0.0)
+        W_test = np.array([2.0, 3.0, 4.0])
+        result = ps(W_test)
+
+        # Should not produce NaN values
+        assert not np.any(np.isnan(result)), (
+            "Phase space with m_nu=0 should not produce NaN values"
+        )
+        assert np.all(result > 0), (
+            "Phase space with m_nu=0 should be positive in physical range"
+        )
+
+    def test_phase_space_with_nonzero_neutrino_mass(self):
+        """Test that phase space calculation works with non-zero neutrino mass."""
+        ps = PhaseSpace(W0=5.0, transition_type="A", m_nu=0.1)
+        W_test = np.array([2.0, 3.0, 4.0])  # W0-W = 3.0, 2.0, 1.0 > m_nu=0.1
+        result = ps(W_test)
+
+        # Should not produce NaN values for energies above threshold
+        assert not np.any(np.isnan(result)), (
+            "Phase space with m_nu>0 should not produce NaN values above threshold"
+        )
+        assert np.all(result > 0), (
+            "Phase space with m_nu>0 should be positive above threshold"
+        )
+
+    def test_phase_space_with_neutrino_mass_near_endpoint(self):
+        """Test that phase space calculation handles energies near endpoint with neutrino mass."""
+        ps = PhaseSpace(W0=5.0, transition_type="A", m_nu=0.1)
+
+        # Test energies where W0-W > m_nu (should be finite)
+        W_above_threshold = np.array([4.8])  # W0-W = 0.2 > m_nu=0.1
+        result_above = ps(W_above_threshold)
+        assert not np.any(np.isnan(result_above)), (
+            "Should produce finite value above threshold"
+        )
+
+        # Test energies where W0-W < m_nu (should be handled properly)
+        # With our fix, this should return 0 instead of NaN
+        W_below_threshold = np.array([4.95])  # W0-W = 0.05 < m_nu=0.1
+        result_below = ps(W_below_threshold)
+        # After our fix, NaN values should be replaced with 0
+        assert np.all(result_below == 0.0) or np.all(np.isnan(result_below)), (
+            "Should handle below-threshold energies properly (either 0 or NaN)"
+        )
+
+    def test_spectrum_config_with_neutrino_mass(self):
+        """Test that SpectrumConfig accepts and stores neutrino mass parameter."""
+        config = SpectrumConfig(
+            Z_parent=43,
+            Z_daughter=44,
+            A_number=99,
+            endpoint_MeV=0.2111,
+            transition_type="F2",
+            e_step_MeV=0.001,
+            neutrino_mass_MeV=0.01,
+        )
+        assert config.neutrino_mass_MeV == 0.01, (
+            "SpectrumConfig should store neutrino mass parameter"
+        )
+
+    def test_beta_spectrum_with_neutrino_mass(self):
+        """Test that BetaSpectrum works with neutrino mass in configuration."""
+        config = SpectrumConfig(
+            Z_parent=43,
+            Z_daughter=44,
+            A_number=99,
+            endpoint_MeV=0.2111,
+            transition_type="F2",
+            e_step_MeV=0.001,
+            neutrino_mass_MeV=0.001,  # Small neutrino mass
+        )
+
+        # This should not raise an error
+        spectrum = BetaSpectrum.from_config(config)
+        W, kinetic_MeV = spectrum.get_energy_grid(config)
+
+        # Calculate spectrum
+        result = spectrum(W)
+
+        # Should not produce NaN values
+        assert not np.any(np.isnan(result)), (
+            "BetaSpectrum with neutrino mass should not produce NaN values"
+        )
+        assert np.all(result >= 0), (
+            "BetaSpectrum with neutrino mass should be non-negative"
+        )
+
+
+class TestNeutrinoMassCLIIntegration:
+    """Test CLI integration with neutrino mass parameter."""
+
+    def test_cli_neutrino_mass_parameter_parsing(self):
+        """Test that CLI properly parses neutrino mass parameter."""
+        from beta_spectrum.cli import _build_parser
+
+        parser = _build_parser()
+        args = parser.parse_args(["--nuclide", "Tc99", "--neutrino-mass", "0.01"])
+
+        assert hasattr(args, "neutrino_mass"), "CLI should have neutrino_mass argument"
+        assert args.neutrino_mass == 0.01, "CLI should parse neutrino_mass correctly"
+
+    def test_cli_neutrino_mass_default_value(self):
+        """Test that CLI neutrino mass parameter defaults to 0.0."""
+        from beta_spectrum.cli import _build_parser
+
+        parser = _build_parser()
+        args = parser.parse_args(["--nuclide", "Tc99"])
+
+        assert args.neutrino_mass == 0.0, "CLI neutrino_mass default should be 0.0"