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

.llm-wiki/wiki/concepts/ssm-spectrum-shape-method.md

@@ -0,0 +1,48 @@
+# SSM — Spectrum Shape Method
+
+## Overview
+
+The Spectrum Shape Method (SSM) is a technique for determining the effective axial-vector coupling constant g_A by comparing theoretically computed β spectral shapes with experimental measurements. It is applicable to forbidden nonunique β decays, where the spectral shape depends nontrivially on g_A through the shape factor C(W; g_A).
+
+## Algorithm
+
+1. **Compute shape factor components** C_V(W), C_A(W), C_VA(W) from nuclear model
+2. **Build theoretical spectrum** for trial g_A values: N(W; g_A) = PhaseSpace × Fermi × ... × C(W; g_A)
+3. **Compare with experimental spectrum** using χ² minimization
+4. **Extract g_A** from the minimum χ² position
+5. **Estimate uncertainty** from χ² = χ²_min + 1
+
+## Key Requirements
+
+- **Forbidden nonunique transition**: The spectral shape must depend on g_A
+- **High-precision measurement**: Low energy threshold (< 10 keV), good energy resolution (< 1 keV)
+- **Accurate nuclear model**: Nuclear shell model (NSM) or MQPM with appropriate valence space
+
+## g_A Values from Literature
+
+| Source | g_A^eff | Method | Notes |
+|---|---|---|---|
+| Paulsen et al. (2024) | 1.526(92) | CVC-fixed sNME | Enhanced value |
+| Song et al. (2025) | 1.0–1.2 | ESSM (farCVC) | Quenched value |
+| Suhonen review | ~0.9 | Half-life comparison | Quenched value |
+
+## Uncertainty Budget
+
+| Component | Uncertainty on g_A | Relative |
+|---|---|---|
+| Fit method (statistics) | 0.0745 | 4.9% |
+| Coulomb displacement energy | 0.0425 | 2.8% |
+| Nuclear model | 0.0294 | 1.9% |
+| Lepton current treatment | 0.0127 | 0.8% |
+| Fit range | 0.0061 | 0.4% |
+| Endpoint energy | 0.0038 | 0.2% |
+| Radiative corrections | 0.0033 | 0.2% |
+| Atomic exchange | 0.0029 | 0.2% |
+| **Combined** | **0.0919** | **6.0%** |
+
+## References
+
+- [[Shape Factor C(W, g_A)]]
+- [[ESSM — Enhanced Spectrum Shape Method]]
+- [[Paulsen et al. (2024)]]
+- [[Song et al. (2025)]]