Hygienic Substitution and Module Linking Implementation

Implements hygienic substitution for TripLang to prevent variable capture, adds a module linker for multi-file programs, and introduces a prelude with Church-encoded arithmetic. The linker resolves cross-module dependencies using Tarjan’s SCC algorithm and emits runnable SKI expressions. Substitution includes alpha-renaming and bound-variable tracking. The CLI adds --link mode for linking .tripc files, and the prelude provides Nat, Bool, and operations like add, mul, eq, pair, fst, snd, cond. Tests cover substitution, linking, CLI, and integration scenarios.