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/nuclear-shell-model-for-decay.md

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+# Nuclear Shell Model for β Decay
+
+## Overview
+
+The Nuclear Shell Model (NSM) is the primary theoretical framework for computing nuclear matrix elements (NMEs) in β decay. For forbidden nonunique decays like ⁹⁹Tc, the NSM computes one-body transition densities (OBTDs) that weight the single-particle transitions contributing to the decay.
+
+## Key Codes
+
+| Code | Description | Valence Spaces |
+|---|---|---|
+| NUSHELLX | Standard NSM code | Multiple (pf, sdg, etc.) |
+| KSHELL | High-performance NSM | Custom valence spaces |
+
+## Valence Spaces for ⁹⁹Tc
+
+### glekpn (Mach interaction)
+- **Protons**: 0f₇/₂, 0f₅/₂, 2p₃/₂, 2p₁/₂, 1g₉/₂
+- **Neutrons**: 0g₉/₂, 0g₇/₂, 1d₅/₂, 1d₃/₂, 2s₁/₂
+
+### jj45pnb (Brown-Lisetskiy interaction)
+- **Protons**: 0f₅/₂, 2p₃/₂, 2p₁/₂, 1g₉/₂
+- **Neutrons**: 0g₇/₂, 1d₅/₂, 1d₃/₂, 2s₁/₂, 0h₁₁/₂
+
+## OBTDs for ⁹⁹Tc
+
+| Transition | OBTD (glekpn) | OBTD (jj45pnb) |
+|---|---|---|
+| 1g₇/₂ → 1g₉/₂ | 0.00994 | 0.01107 |
+| 2d₅/₂ → 1g₉/₂ | 0.47752 | −0.43985 |
+
+**Key insight**: The dominant transition is 2d₅/₂ → 1g₉/₂ with OBTD ≈ 0.478. The jj45pnb model gives opposite sign for this transition.
+
+## sNME and CVC
+
+The small relativistic vector NME (sNME) is related to the large vector NME via the CVC hypothesis:
+
+```
+sNME = (R/√10) · [W₀ − (m_n − m_p) + E_C] · l-NME
+```
+
+where E_C is the Coulomb displacement energy. For ⁹⁹Tc, [W₀ − (m_n − m_p)] = −0.487 MeV (small Q-value makes this critical).
+
+## References
+
+- [[Shape Factor C(W, g_A)]]
+- [[SSM — Spectrum Shape Method]]
+- [[Ramalho & Suhonen (2024)]]
+- [[Song et al. (2025)]]