Sum Types Implementation - 3-4 Week Plan

Architecture: Extend existing plugin system with SumTypesPlugin that transforms enum declarations into tagged unions with zero runtime overhead. Parser will recognize enum syntax with unit/tuple/struct variants, AST nodes will represent sum types natively, and exhaustiveness checker will validate pattern matches at compile time. Transpiles to idiomatic Go code using tag enums + structs with pointer fields, constructor functions, and switch statements for pattern matching. Supports generics via Go 1.18+ generic types. Six-phase implementation: (1) AST & Parser, (2) Basic Transpilation, (3) Pattern Matching, (4) Exhaustiveness Checking, (5) Generics Support, (6) Polish & Optimization. Critical foundation for Result<T,E> and Option<T> types.
