docs: prune research vault to theory-only scope

Remove gA-extraction notes, nuclear-structure notes, paper-summary
refs, detector-response correction note and the finished test
migration checklist. Rewrite the vault index for the single-package
theory calculator; repoint dangling wiki-links in surviving notes to
inline citations; refresh docs_check.py (drop bs_exp CLI check,
register removed files).

Literature and nuclear-structure research resumes in dedicated
projects; the removed notes are recoverable from history.

TODO.md

@@ -13,28 +13,28 @@ ______________________________________________________________________
 Single distributable, `beta_spectrum`:
 
 ```
-beta_spectrum/              ← theory-only core
-├── __init__.py
-├── base.py                 ← SpectrumComponent base
-├── constants.py            ← physical constants
-├── utils.py                ← T_to_W, W_to_T, momentum
-├── spectrum.py             ← BetaSpectrum, SpectrumConfig, BranchConfig
-├── nuclear_data.py         ← paceENSDF integration + JSON input
-├── cli.py                  ← bs_pnpi command-line interface
-├── logging_utils.py        ← structured logging helpers
-├── components/             ← individual correction factors
-│   ├── phase_space.py
-│   ├── fermi.py
-│   ├── finite_size.py
-│   ├── screening.py
-│   ├── exchange.py
-│   └── radiative.py
-├── data/
-│   └── exchange_coeff.csv  ← X(Z,W) fit coefficients, Z=2..120
-└── output/                 ← visualization, CSV export
-    ├── core.py             ← BetaSpectrumAnalyzer
-    ├── csv_export.py       ← CSVEmitter
-    └── visualization.py    ← Visualizer, parse_spectrum_csv
+beta_spectrum/               ← theory-only core package
+│   ├── __init__.py          ← public API
+│   ├── base.py              ← SpectrumComponent base
+│   ├── constants.py         ← physical constants
+│   ├── utils.py             ← T_to_W, W_to_T, momentum
+│   ├── spectrum.py          ← BetaSpectrum, SpectrumConfig, BranchConfig
+│   ├── nuclear_data.py      ← paceENSDF integration + JSON input
+│   ├── cli.py               ← bs_pnpi command-line interface
+│   ├── logging_utils.py     ← structured logging helpers
+│   ├── components/          ← individual correction factors
+│   │   ├── phase_space.py
+│   │   ├── fermi.py
+│   │   ├── finite_size.py
+│   │   ├── screening.py
+│   │   ├── exchange.py
+│   │   └── radiative.py
+│   ├── data/
+│   │   └── exchange_coeff.csv
+│   └── output/              ← visualization, CSV export subpackage
+│       ├── core.py          ← BetaSpectrumAnalyzer
+│       ├── csv_export.py    ← CSVEmitter
+│       └── visualization.py ← Visualizer, parse_spectrum_csv
 ```
 
 ### Rationale