tatara-lisp — a small homoiconic S-expression language.
The surface is homoiconic: the reader produces an AST (Sexp) that is
itself S-expressions. Macros operate on Sexp and yield Sexp.
Scope of v0 (this scaffold): lexer, reader, Sexp AST, environment,
plus a minimal evaluator shell (special forms quote, if, let, lambda).
The ProcessSpec compiler (defpoint macro + flattening to
tatara_process::ProcessSpec) lands in compile.rs in the next pass.