papers: sync v2 of Papers A and B from private; retire v3.3 predecessors

Adds the April 2026 v2 versions of the public companion papers, replacing
the v3.3-era ancestors which are now archived in private/legacy/.

Added (v2):
  markdown/g2_certified_neck.md           (Paper A — Zenodo 18860358)
  tex/g2_certified_neck.tex
  tex/g2_certified_neck.bib
  pdf/g2_certified_neck.pdf

  markdown/g2_spectral.md                 (Paper B — Zenodo 18920368)
  tex/g2_spectral.tex
  tex/g2_spectral.bib
  pdf/g2_spectral.pdf

Removed (v3.3 predecessors, same Zenodo DOIs, same papers superseded):
  markdown/Explicit_G2_Metric.md
  tex/Explicit_G2_Metric.tex
  pdf/Explicit_G2_Metric.pdf
  markdown/Spectral_Geometry.md
  tex/Spectral_Geometry.tex
  pdf/Spectral_Geometry.pdf

Substantive v3.3 → v2 changes (recap from private CHANGELOG):
- Title: "approximate / compact" → "certified torsion-free / TCS neck model"
- Math correction Paper B: G_IJ (L² Gram, pos def) vs I_IJ (K3 cup product,
  signature (3,18) restriction of K3 lattice (3,19))
- Compartmentalization: speculative GIFT physics removed
- λ₁ = 6π²/475 reframed as derived a posteriori (not topological prediction)
- T² coordinate ψ → χ (ψ reserved for coassociative form ψ = ⋆φ)
- Conditional compact-extension framing throughout
- New companion citations (Zhou, cymyc, Ishige-Takayasu, Nilsson, etc.)
- Letter→numbered theorem labels harmonized

GIFT v3.3 framework files (gift_v3.3_main/S1/S2) untouched — they are a
distinct paper (the framework itself), kept as the published v3.3 mirror
pending the v3.4 framework draft sync.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Brieuc