Implement SQLRules 2.0 release phase

[eddiethedean]

Summary

• Introduce RuleSchema as a full Pydantic model restricted to SQL-compilable declarations, plus from_pydantic conversion reports.
• Implement lax and strict scalar rules, backend-safe preparation and capability checks, grouped total predicates, CompiledRules, and notwhere().
• Upgrade core and all four providers to 2.0.0 / plugin API v2; add a database execution corpus and CI services.
• Refresh 2.0 docs, migration guide, examples, benchmark, and support matrix.

Verification

• 51 passed, 20 database tests skipped locally without service URLs.
• 81.15% local statement and branch coverage (80% CI floor).
• Core and provider mypy, Ruff, Sphinx, all five package builds and Twine checks, installed wheel smoke, examples, and version synchronization passed.
• GitHub Actions passed the live PostgreSQL, MySQL, and SQL Server conformance corpus, plus Python 3.10–3.13, docs, lint, and package jobs.