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

docs/nuclear-structure/01-shape-factors.md

@@ -89,5 +89,39 @@ The nuclear structure analogue of the electrostatic deformation correction ([[co
 
 ## Calculator Status
 
-> [!info] Not yet implemented
-> The shape factor $C(Z,W)$ is NOT part of the current calculator. This note documents the theory for future implementation. For superallowed Fermi transitions, $C(Z,W) = |M_F|^2 \approx 1$ (isospin symmetry), so it can be approximated as unity in many cases.
+> [!info] Implementation Planned
+> The shape factor $C(Z,W; g_A)$ is being implemented as `ShapeFactorGA` component. See `.rpiv/artifacts/research/2026-05-27_gA-spectrum-shape-factor-implementation.md` for the implementation plan.
+
+## Latest Research (May 2026)
+
+### Key Findings from Paulsen et al. (2024) Fig. 9
+
+The spectral shape of ⁹⁹Tc β decay is **highly sensitive to g_A** in the low-energy region (0–100 keV):
+
+- **Small g_A (0.5–1.0)**: Bell-shaped spectrum with low-energy enhancement
+- **Large g_A (1.4–2.0)**: Flattened spectrum with low-energy suppression
+- **Crossover point**: All spectra intersect around 100 keV
+
+### g_A Values — Resolved
+
+| Source | g_A^eff | Method | Status |
+|---|---|---|---|
+| Paulsen et al. (2024) | 1.526(92) | CVC-fixed sNME | Enhanced (controversial) |
+| Song et al. (2025) | 1.0–1.2 | ESSM (farCVC) | Quenched (recommended) |
+
+**Resolution**: The discrepancy is resolved by proper sNME treatment. ESSM (sNME as fit parameter) gives quenched g_A = 1.0–1.2, consistent with other forbidden decays.
+
+### Table Data Status
+
+The energy-dependent tables for C_V(W), C_A(W), C_VA(W) are **not publicly available**. Priority actions:
+1. Obtain from supplementary materials of Kostensalo & Suhonen (2017) or Ramalho & Suhonen (2024)
+2. Contact Suhonen group (jouni.t.suhonen@jyu.fi) for table data
+3. Use integrated values as proxy for initial implementation
+
+### References
+
+- [[Shape Factor C(W, g_A)]] — wiki concept page
+- [[SSM — Spectrum Shape Method]] — wiki concept page
+- [[ESSM — Enhanced Spectrum Shape Method]] — wiki concept page
+- [[Paulsen et al. (2024)]] — wiki entity page
+- [[Song et al. (2025)]] — wiki entity page