Fix SPSS VLS compatible names

[TonisOrmisson]

What changed

• parse and rebuild SPSS subtype-14 compatible-name records strictly
• replace compatible names atomically and fail closed on malformed or duplicate metadata
• keep ZSAV offsets stable and expose a shared var_compat_names comparator
• add shared fixtures, regression tests, and documentation
• verify the fix together with the merged SQL transformation workflow

Why

Subtype-14 long-variable-name metadata must stay consistent when variables are normalized or rewritten. Stale or malformed records can otherwise make SAV and ZSAV files disagree about variable identity.

Validation

• focused subtype-14 regression tests
• full combined non-service suite: 129 passed
• GitHub Actions matrix rerun on the current main integration
• independent blocker-only review completed with no remaining findings

Closes #2