feat(ext): add DuckDB support

[timveil]

Companion to #445 (new database support). Top modern pick from the mid-2026 research.

Why

DuckDB is the fastest-rising analytical database (DB-Engines #81→#51 in a year; #4 in the Stack Overflow survey), MIT-licensed, with an official JDBC driver (org.duckdb:duckdb_jdbc, 1.5.3.0 as of May 2026). It supports real PK/FK constraints, so Bloviate's FK dependency-graph fill works as-is.

Big test-harness win: DuckDB is embedded/in-process (jdbc:duckdb: file or :memory:), so its integration tests need no Docker/TestContainers — faster CI, and it doubles as a fixture for the testing-framework work (#448).

Scope — new dialect (own support class)

• DuckDBSupport extends AbstractDatabaseSupport with a configure() override; do the empirical DATA_TYPE/TYPE_NAME discovery first (per the feat(ext): full PostgreSQL and MySQL vendor-type support #444 method) against duckdb_jdbc.
• Types to cover: BOOLEAN, TINYINT/SMALLINT/INTEGER/BIGINT/HUGEINT and unsigned UTINYINT…UBIGINT, FLOAT/DOUBLE, DECIMAL, VARCHAR, BLOB, DATE/TIME/TIMESTAMP/TIMESTAMPTZ/INTERVAL, UUID, JSON, and the composite types LIST/ARRAY, STRUCT, MAP, ENUM, BIT. Reuse existing generators (UUIDGenerator, JsonbGenerator, array generators, BitStringGenerator); decide whether to support the composite types (STRUCT/MAP) initially or throw UnsupportedOperationException for them.
• forProduct: add a duckdb branch + selection test.
• Test base without TestContainers (plain JDBC URL); create_tables.duckdb.sql + fillTestTables.
• Driver: org.duckdb:duckdb_jdbc (test scope).

Acceptance criteria

• DuckDBSupport registered and selectable
• Embedded (no-Docker) integration test fills a comprehensive type schema cleanly
• Composite/unsupported types either generated or cleanly rejected
• README "Database Support" table updated