Sum Types Phase 2.5 Completion Plan

This plan completes the Sum Types implementation by fixing the P1 position information bug (blocking golden tests), implementing pattern destructuring for match expressions, and addressing 10 IMPORTANT code review issues. The previous session achieved 100% unit test pass rate (31/31) but left integration testing blocked and core match functionality incomplete. This 2-3 day effort will deliver production-ready sum types with full end-to-end verification, enabling merge to main and unblocking Phase 3 development.
