feat: implement opt-in official SPSS Frontend 0.3

[TonisOrmisson]

Alignment block 2, stacked after #31. Explicit official Frontend 0.3 request and live apply selection over exact Plan 0.1/0.2. Default behavior and Python-owned schema extension remain separate. Includes strict parser/request validation, metadata-aware lowering, pre-compilation dataset lock and MySQL/MariaDB READ COMMITTED to prevent stale additive-label replacement.

Validation: 90/90 effective official cases; 590 passed, 9 skipped locally; compileall/diff check passed. All 14 final-commit CI jobs passed at 10433d9, including MySQL/MariaDB interleaved-label regression: https://github.com/OpenStatSpec/python/actions/runs/34484267005

Read-only review findings fixed; final production-fix review: No findings. Final follow-up selects official frontend in the service regression. Additional 37-program Python/PHP parity check: no canonical-plan/diagnostic differences.

Merge #31 first. No version, dependency, pin change or publication.

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