Validate schema compatibility and retain runtime gates

[philipgreat]

Closes #200.

What changed

• fail closed when existing SQL columns conflict with the model type, nullability, VARCHAR length, or decimal value domain
• propagate required metadata through GenericSQLProperty
• expose compatible shape metadata from SQLite, MySQL, PostgreSQL, and DuckDB adapters
• retain bounded optimistic-contention and real SQLite multi-connection ID allocation tests
• add DuckDB JDBC integration coverage and a two-example runtime gate

Compatibility

Providers that only report column names continue to work; value-domain checks activate only when usable type metadata is present. This preserves the current context-owned schema and indexExists contracts rather than restoring recovery-era APIs.

Verification

• mvn -q -pl teaql-sql-portable,teaql-sqlite,teaql-mysql,teaql-postgres,teaql-duckdb -am test
• DuckDB integration gate with JDBC 1.0.0 and 1.5.5.1
• SQLite ID concurrency gate repeated four times
• examples/verify-runtime-examples.sh: 2/2 passed