WIT generation for the Fidius WASM backend (FIDIUS-I-0023).
Maps the Rust types in a #[plugin_interface] to WIT, per
docs/explanation/wasm-component-abi.md. Primitives, String, Vec<T>,
Option<T>, and Result<T, PluginError> map directly; user types that
carry #[derive(WitType)] map to WIT records (structs) and variants
(enums), referenced by their kebab-case name.
This is a plain library (not a proc-macro), so fidius-macro, the build.rs
helper, and the fidius wit CLI can all share one implementation.