Parse an Aver type annotation string into a Type.
Returns Type::Unknown for unknown identifiers (internal fallback).
Prefer parse_type_str_strict for user-facing type annotations.
Parse a type annotation string strictly.
Returns Err(unknown_name) if the string is a non-empty identifier
that does not map to a known type (i.e. a likely typo).
Generic forms (Result<...>, Option<...>, List<...>) with valid inner types are accepted.