Const-constructible type definitions for Harn builtin signatures.
Both harn-parser (for typechecking) and harn-vm (for runtime metadata)
consume these shapes. Living in a dep-free crate lets the parser see the
types without depending on the VM, and lets the #[harn_builtin] proc-macro
emit const literals that link into either side.
Ty::to_type_expr and friends, which convert into the parser's runtime
TypeExpr, live in harn-parser since they depend on parser-internal AST.
The [shapes] submodule holds the named structural-record consts
(LLM_CALL_OPTIONS, LLM_CALL_RESULT, TRANSCRIPT, …) shared by the
parser's static typechecking tables and the #[harn_builtin] macro's
@NAME signature injection.