feat: multi-dialect SQL support (MySQL, SQLite, DuckDB, BigQuery)

[richardwooding]

Summary

• Add pluggable Dialect interface (~40 methods) enabling cel2sql to generate SQL for PostgreSQL, MySQL, SQLite, DuckDB, and BigQuery from a single CEL expression
• Add per-dialect type providers with LoadTableSchema support for runtime schema introspection from live databases (MySQL via information_schema, SQLite via PRAGMA table_info, DuckDB via information_schema, BigQuery via client API)
• Add per-dialect index advisors generating dialect-appropriate DDL recommendations (BTREE/GIN/trgm for PG, BTREE/FULLTEXT for MySQL, BTREE for SQLite, ART for DuckDB, CLUSTERING/SEARCH_INDEX for BigQuery)
• Add shared test infrastructure (testcases/, testutil/) with per-dialect expected SQL and a multi-dialect test runner
• Add integration tests for SQLite (in-memory), MySQL (testcontainer), and BigQuery (testcontainer emulator)
• Extract dialect-agnostic schema types into schema/ package
• Update README with multi-dialect documentation, dialect comparison table, CGO warning for DuckDB, and expanded Requirements section

Test plan

• make ci passes (fmt, lint, test, vuln-check)
• All existing PostgreSQL tests continue to pass (backward compatibility)
• New per-dialect unit tests pass for MySQL, SQLite, DuckDB, BigQuery
• Integration tests pass: SQLite (in-memory), MySQL (testcontainer), BigQuery (emulator, amd64 only)
• Provider tests validate type mapping for each dialect
• Index advisor tests verify DDL generation per dialect

🤖 Generated with Claude Code

[derekperkins]

Fixes #102