Port recovered schema compatibility and runtime gates #200

Problem

A recovery audit found Java runtime work that was not present on the current main line:

• Ensure Schema did not validate an existing column type, nullability, or bounded text/decimal value domain.
• GenericSQLProperty did not propagate required metadata to SQLColumn.
• The portable ID allocator lacked retained SQLite multi-connection and bounded-contention evidence.
• DuckDB and the retained runtime examples lacked executable repository gates.

Scope

Port only the valuable capabilities onto the current main architecture. Keep the current indexExists and context-owned schema contracts; do not restore obsolete recovery APIs.

Acceptance

• Existing incompatible columns fail with entity, table, column, dialect, expected, and actual context.
• Missing optional provider metadata remains backward compatible.
• SQLite concurrent ID allocation produces no duplicates and respects bootstrap floors.
• DuckDB 1.0.0 and current JDBC pass CRUD, query, schema, facet/window, and mutation checks.
• Conformance and School runtime examples pass from one repository script.