db2: add integration test infrastructure and macOS native runner

Shared test helpers (db2test package) manage the DB2 Docker container
lifecycle for integration tests, including ASNCDC pre-warm and
per-test table cleanup. TestMain in the integration suite shares a
single container across all tests to avoid 8-minute Docker init
overhead. The macOS runner script builds a native test binary that
uses the local IBM clidriver dylib instead of a Linux container,
enabling fast local iteration on Apple Silicon.

Integration tests cover: snapshot+streaming, multi-chunk incremental
snapshot, concurrent INSERT/UPDATE/DELETE during snapshot, and
resume-after-crash checkpoint recovery.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: emaxerrno