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_phase_space.py

@@ -74,15 +74,15 @@ class TestPhaseSpaceNeutrinoMass:
         val_massive = ps_massive(W_near_endpoint)[0]
 
         assert not np.isnan(val_massive), "Should produce finite value"
-        assert (
-            val_massive < val_zero
-        ), "Massive neutrino must suppress spectrum near endpoint"
+        assert val_massive < val_zero, (
+            "Massive neutrino must suppress spectrum near endpoint"
+        )
 
     @pytest.mark.physics
     def test_massive_neutrino_below_threshold(self):
-        """With m_nu > 0, below the effective threshold (W₀−W < m_nu), sqrt gives NaN.
+        """With m_nu > 0, below the effective threshold (W₀−W < m_nu), the spectrum should be 0.
 
-        This is expected physical behavior — the spectrum simply doesn't exist there.
+        This is the correct physical behavior — the spectrum doesn't exist below the kinematic threshold.
         """
         ps = PhaseSpace(W0=5.0, m_nu=0.1)
 
@@ -94,6 +94,4 @@ class TestPhaseSpaceNeutrinoMass:
         W_deep_below = np.array([4.95])  # W₀ − W = 0.05 < m_nu=0.1 — below threshold
         result_below = ps(W_deep_below)
 
-        assert np.isnan(
-            result_below[0]
-        ), "Below kinematic threshold, spectrum should be NaN"
+        assert result_below[0] == 0.0, "Below kinematic threshold, spectrum should be 0"