Tags give the ability to mark specific points in history as being important
feat: Unified PushdownPlan architecture (Phases 1-4) - PushdownPlan dataclasses for query representation - Unified plan builder in PlanExecutor - execute_pushdown() in SparkGraph - Wired up with fallback to legacy methods Ready for Databricks validation before Phase 5 cleanup.
v0.2.39 - Physical layer with index-free adjacency