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.

docs/02-phase-space.md

@@ -42,7 +42,7 @@ $$K(W) = \sqrt{\frac{N(W)}{p W F(Z,W)}} \approx W_0 - W \quad (\text{linear for
 Deviations from linearity near the endpoint indicate:
 - Non-zero neutrino mass
 - Weak magnetism / recoil effects ([[corrections/06-recoil]])
-- New physics (Fierz term [[Hayen2017_summary#The Fierz Interference Term]])
+- New physics (Fierz interference term; see Hayen et al., *Rev. Mod. Phys.* **90**, 015008 (2018))
 
 ## Calculator Implementation
 
@@ -62,7 +62,7 @@ The `PhaseSpace` calculator accepts a `transition_type` parameter that determine
 | `F2U`, `F3` | 2, 3 | $p_\nu^4 + \frac{3}{10}p_\nu^2 p_e^2 + p_e^4$ |
 | `F3U`, `F4` | 3, 4 | $p_\nu^6 + 7p_\nu^4 p_e^2 + 7p_\nu^2 p_e^4 + p_e^6$ |
 
-The forbidden factor modifies the energy dependence of the spectrum, becoming significant at higher electron energies. See `[[nuclear-structure/01-shape-factors]]` for the full shape factor treatment.
+The forbidden factor modifies the energy dependence of the spectrum, becoming significant at higher electron energies. The full nuclear shape factor $C(Z,W)$ is external nuclear-structure theory, out of scope for this repository: the calculator treats the forbiddenness classes above as input configuration.
 
 ## Energy Range