docs: restructure knowledge base into tree layout + gA extraction module

- Organize into tree structure: corrections/, nuclear-structure/, gA-extraction/, refs/
- Move all correction notes to corrections/ (01-09)
- Move nuclear structure to nuclear-structure/ with index
- Add gA-extraction/ with SSM algorithm, sNME/ESSM formalism, Tc99 case study, and implementation TODO
- Reorganize refs by topic (electromagnetic/, forbidden-decay/)
- Update all wikilinks to new paths
- Remove radiative_spec.md (code algorithm, not reference)
- Lint: all wikilinks verified valid
- Comprehensive gA extraction research based on Paulsen et al. (2024) and Song et al. (2025)

docs/nuclear-structure/index.md

@@ -0,0 +1,29 @@
+---
+title: "Nuclear Structure — Overview"
+date: 2026-05-14
+tags:
+  - beta-spectrum/nuclear-structure
+status: active
+aliases: [Nuclear Theory, Shape Factor, NME]
+---
+
+# Nuclear Structure Effects in β Decay
+
+## Overview
+
+Nuclear structure enters the β spectrum through the **shape factor** $C(Z,W)$, which encodes how the nuclear wave functions and transition matrix elements modify the spectrum beyond the point-nucleus Coulomb interaction.
+
+## Contents
+
+- [[nuclear-structure/01-shape-factors]] — Shape factor $C(Z,W)$: decomposition, multipole expansion, impulse approximation, induced currents
+
+## Key Concepts
+
+- **Shape factor decomposition:** $C(W) = g_V^2 C_V(W) + g_A^2 C_A(W) + g_V g_A C_{VA}(W)$
+- **Forbidden decays:** Nonunique transitions are sensitive to $g_A$ → basis for SSM
+- **sNME:** Small relativistic vector NME — critical for precision but hard to compute
+
+## Related
+
+- [[gA-extraction/01-ssm-algorithm]] — How to extract $g_A$ from shape factor
+- [[corrections/06-recoil]] — Weak magnetism, induced currents, recoil effects