Ẑ-EXT: extend the Ẑ-invariant across the genus-decorated-vertex and b₁>0 walls #4

Ledger: notes/QUESTIONS-2026-07-19.md §I item 6 (phase 87)

Extend the Ẑ-invariant across the genus-decorated-vertex and b₁ > 0 walls, with the spin refinement as the frontier.

Sealed checkpoint. The carrier's DTAB tables (data/DTAB-DATA-2026-07-19.md) are the pre-computed checkpoint any extension must reproduce.

Relevant material: phases/phase87-GAP-Zhat.md, data/DTAB-PAPER.md.