Phase 4.2 architecture plan complete. Designed four enhancements (guards, Swift syntax, tuples, enhanced errors) that reuse 90% of Phase 4.1 infrastructure with minimal risk. Implementation sequence defined: 4 phases, 5-6 hours total, targeting 76 tests passing (57 existing + 19 new). Enhanced errors use rustc-style source snippets, tuple exhaustiveness uses decision tree algorithm, guards use goto labels for fast fallthrough. All backward compatible with Phase 4.1. 8 design decisions need user input (tuple limits, guard semantics, Swift syntax details).
