Add DBD::Pg compatibility shim backed by JDBC

[fglock]

Summary

Add a PerlOnJava DBD::Pg compatibility shim backed by the existing
DBD::JDBC infrastructure and a PostgreSQL JDBC driver.

Motivation

PerlOnJava already provides DBI through JDBC and has a working DBD::JDBC
base driver plus a DBD::SQLite DSN-translation shim. However, CPAN modules
that use the conventional dbi:Pg: DSN or declare DBD::Pg currently cannot
use PostgreSQL through PerlOnJava without a native libpq/XS driver.

MetaCPAN lists 83 distributions depending directly on DBD::Pg, including
recent and actively maintained projects such as:

• App::Netdisco
• DBIx::Auto::Migrate
• Lilith
• DBIO::PostgreSQL
• Mojo::Pg
• Yote::SQLObjectStore
• DBD::Patroni
• IO::Async::Pg
• Tapper::Schema

Reverse-dependency evidence:
https://metacpan.org/module/DBD%3A%3APg/requires

Proposed scope

• Add src/main/perl/lib/DBD/Pg.pm as a PerlOnJava compatibility driver.
• Inherit the common handle implementation from DBD::JDBC.
• Translate common DSNs such as:
  • dbi:Pg:dbname=name
  • dbi:Pg:dbname=name;host=host;port=5432
    into PostgreSQL JDBC URLs.
• Preserve standard DBI operations: connect, prepare, execute, fetch,
  transactions, metadata, quoting, and error handling.
• Keep the PostgreSQL JDBC connector optional and use the existing
  Configure.pl/classpath driver configuration path.
• Add focused DSN and handle tests without requiring a live server, plus an
  optional integration suite against PostgreSQL when a JDBC driver and server
  are available.

PostgreSQL-specific compatibility

The first milestone should target ordinary DBI consumers. Follow-up work may
be needed for:

• PostgreSQL arrays, jsonb, bytea, and other type mappings.
• COPY operations.
• Notifications and asynchronous queries.
• Server-side prepare behavior.
• pg_* database and statement-handle methods.
• PostgreSQL-specific SSL, socket, and connection attributes.

Unsupported PostgreSQL-specific behavior should be reported clearly rather
than silently emulated incorrectly. The shim should not perform broad SQL
rewriting; PostgreSQL SQL semantics should come from the PostgreSQL JDBC
driver.

Acceptance criteria

• use DBD::Pg succeeds under PerlOnJava without XS loading.
• Standard DBI->connect('dbi:Pg:...', ...) reaches the PostgreSQL JDBC
  driver.
• A PostgreSQL-backed smoke test exercises schema introspection, prepared
  statements, transactions, parameter binding, and error propagation.
• JVM and interpreter backends have equivalent behavior for the shim.
• At least one representative CPAN consumer can connect using its normal
  DBD::Pg DSN once PostgreSQL and the JDBC driver are supplied.