Mastering the Expand and Contract Pattern for Zero-Downtime Database Migrations

Act as a Senior Database Reliability Engineer. Design a highly technical, uncompromising strategy for implementing the 'Expand and Contract' (Parallel Changes) architectural pattern to execute a highly destructive [SCHEMA_CHANGE_DETAIL] (e.g., splitting a column or changing a data type) on the massive [TABLE_NAME] living within a high-throughput [DATABASE_ENGINE] environment. Your master plan must flawlessly orchestrate four distinct, zero-downtime stages: 1) Expansion (adding the new non-blocking structures), 2) Migration (background batch-copying of data via triggers/scripts), 3) Transition (deploying the new code), and 4) Contraction (safely dropping the legacy structures). Seamlessly integrate this multi-step logic into a [CI_CD_PLATFORM] pipeline, providing the exact, non-locking SQL execution templates and the programmatic logic required for automated data-parity validation between the old and new schemas before the final contract phase.