WCAB 0.8.0 — external workbook-link policy coverage

WCAB 0.8.0 adds a deterministic external-workbook link startup-policy case. The synthetic external formula and local downstream reference stay unchanged while workbookPr/@updateLinks moves from never to always. The source workbook is deliberately absent: the benchmark validates stored OOXML and formula text only; it does not resolve, refresh, authenticate to, or calculate the source.\n\nValidation: 23 cases, 25 facts, 37 tests on Python 3.12/3.13 locally; hosted CI passed on Python 3.10/3.13. The optional FormulaFence reference pass matched all 24 mappable facts and all three coverage declarations; one structural-equivalence annotation remains intentionally unmapped.