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/shape-factor-cw-ga.md

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+# Shape Factor C(W, g_A)
+
+## Overview
+
+The g_A-dependent nuclear shape factor C(W, g_A) is the key nuclear structure quantity that encodes the energy-dependent modification of the β decay spectrum due to forbidden nonunique transitions. For ⁹⁹Tc (second-forbidden nonunique, 9/2₁⁺ → 5/2₁⁺, Q ≈ 296 keV), the shape factor decomposes as:
+
+```
+C(W; g_A) = g_V² · C_V(W) + g_A² · C_A(W) + g_V · g_A · C_VA(W)
+```
+
+with g_V = 1.0 fixed by CVC.
+
+## Components
+
+| Component | Description | Sign for ⁹⁹Tc | Magnitude |
+|---|---|---|---|
+| C_V(W) | Vector contribution | + | 2.240 × 10⁻⁹ (integrated) |
+| C_A(W) | Axial-vector contribution | + | 2.130 × 10⁻⁹ (integrated) |
+| C_VA(W) | Vector-axial interference | − | −4.361 × 10⁻⁹ (integrated) |
+
+**Key insight**: C_VA is negative and twice as large as C_V or C_A. The total integrated shape factor is two orders of magnitude smaller due to destructive interference.
+
+## g_A Dependence
+
+The spectral shape is most sensitive to g_A in the **0–100 keV range** (low-energy region). Figure 9 from Paulsen et al. (2024) shows:
+
+- **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
+
+## Table Data Sources
+
+| Source | Method | Coverage | Availability |
+|---|---|---|---|
+| Kostensalo & Suhonen (2017) PRC 96, 024317 | MQPM | 26 forbidden decays | Supplementary material (not public) |
+| Ramalho & Suhonen (2024) | NSM (NUSHELLX) | ⁹⁹Tc-specific | Supplementary material (not public) |
+| Song et al. (2025) arXiv:2506.17544 | ESSM + NSM/MQPM | ⁹⁹Tc | Integrated values only |
+
+## Implementation
+
+The ShapeFactorGA component should:
+1. Load C_V(W), C_A(W), C_VA(W) tables from CSV files
+2. Interpolate using cubic spline (scipy.interpolate.CubicSpline)
+3. Compute C(W; g_A) = C_V + g_A²·C_A + g_A·C_VA
+4. Return the shape factor for the full spectrum calculation
+
+## References
+
+- [[SSM — Spectrum Shape Method]]
+- [[Nuclear Shell Model]]
+- [[CVC Hypothesis]]
+- [[Paulsen et al. (2024)]]
+- [[Song et al. (2025)]]