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/essm-enhanced-spectrum-shape-method.md

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+# ESSM — Enhanced Spectrum Shape Method
+
+## Overview
+
+The Enhanced Spectrum Shape Method (ESSM) extends the standard SSM by treating the small relativistic vector nuclear matrix element (sNME) as a free parameter alongside g_A. This allows simultaneous fitting of both the spectral shape AND the experimental half-life.
+
+## Motivation
+
+The sNME is difficult to compute accurately in nuclear shell model because it involves contributions outside the valence shell. The CVC prediction gives a "canonical" value, but this assumes a perfect many-body theory — which finite valence spaces are not.
+
+## Algorithm
+
+1. Fix g_A at a trial value
+2. Vary sNME to simultaneously fit the experimental half-life
+3. Because the half-life depends quadratically on sNME, there are typically **two solutions** for each g_A:
+   - **sNME(c)**: closer to the CVC value (closeCVC)
+   - **sNME(f)**: further from the CVC value (farCVC)
+4. Compare both solutions' spectral shapes with the data
+
+## sNME Values for ⁹⁹Tc
+
+| Model | g_A^eff | sNME(c) [fm³] | sNME(f) [fm³] | CVC sNME [fm³] |
+|---|---|---|---|---|
+| jj45pnb | 1.0 | 0.0666 | −0.0687 | −0.1705 |
+| glekpn | 1.0 | 0.1140 | −0.0202 | −0.0080 |
+
+## Results from Song et al. (2025)
+
+- **Best fit**: glekpn with g_A^eff = 1.2, sNME(f) = −0.0698 fm³, χ²_ν = 1.066
+- **Conclusion**: Quenched g_A = 1.0–1.2 with farCVC sNME provides the best agreement with data
+
+## References
+
+- [[SSM — Spectrum Shape Method]]
+- [[Shape Factor C(W, g_A)]]
+- [[Song et al. (2025)]]
+- [[Kumar et al. (2021)]]