Summary

• Make cst and lexer submodules private, re-export public items through parser/mod.rs
• Keep ast as public submodule (meaningful namespace for AST types)
• Update import paths across codebase to use the facade

Why

Enforces the facade pattern: parser/mod.rs is now the sole re-export hub for external consumers. Internal file structure (cst.rs, lexer.rs) is hidden, enabling future refactoring without breaking external imports.