Split JDBC plugin into dialect modules with Jdbi and Hikari-backed access

[me0106]

Summary

This PR restructures the JDBC plugin into a real parent module with separate dialect implementations:

• Converts memind-plugin-jdbc into a parent POM under memind-plugins/memind-plugin-jdbc
• Adds separate JDBC modules:
  • memind-plugin-jdbc-core
  • memind-plugin-jdbc-sqlite
  • memind-plugin-jdbc-mysql
  • memind-plugin-jdbc-postgresql
• Removes the old JdbcStore facade and DriverManagerDataSource
• Uses HikariDataSource directly for plain Java JDBC access
• Simplifies JDBC persistence around Jdbi-based access patterns
• Removes the extra JdbcExecutor wrapper
• Moves dialect SQL and concrete implementations into each dialect module
• Updates the Spring Boot JDBC starter to create JdbcMemoryAccess through dialect plugin factories
• Updates examples and README usage from JdbcStore to direct dialect factories

DataSource Ownership

Plain Java usage creates a HikariDataSource through the dialect plugin factory and the returned JdbcMemoryAccess owns
that pool.

Spring Boot usage relies on Boot-created DataSource / HikariDataSource; the JDBC starter consumes the provided pool
and does not create its own.

Testing

• mvnd -pl :memind-example-java -am package
• /opt/homebrew/bin/mvnd clean package
• git diff --check

Full reactor build passed successfully.