lib/ocaml/runtime.sx defines the stdlib in OCaml syntax (not SX): every
function exercises the parser, evaluator, match engine, and module
machinery built in earlier phases. Loaded once via ocaml-load-stdlib!,
cached in ocaml-stdlib-env, layered under user code via ocaml-base-env.
List: length, rev, rev_append, map, filter, fold_left/right, append,
iter, mem, for_all, exists, hd, tl, nth.
Option: map, bind, value, get, is_none, is_some.
Result: map, bind, is_ok, is_error.
Substrate validation: this stdlib is a nontrivial OCaml program — its
mere existence proves the substrate works.
