Add `Metadata<B>` trait and `#[metadata(Entry)]` attribute macro
to attach arbitrary, compile-time-typed metadata to entries.
Implement `ProgramCollect::get_metadata<T>()` for runtime retrieval,
backed by a global registry populated by `register_metadata!`.

Extend `pathf` with `MetadataPattern` to resolve metadata types
across modules at build time. Add two examples demonstrating
metadata usage with and without pathf integration.

Previously, #[command] required at least one owned parameter for
args injection.  Now, if the first parameter is a reference or
there are no parameters, a synthetic entry parameter is inserted
so the function can still be used with #[chain].

Add a new `#[command]` attribute macro (feature-gated behind
`extra_macros`) that converts a plain function with a `Vec<String>`
parameter into a fully wired Mingling command. The macro generates a
`dispatcher!` call, a `#[chain]` wrapper, and a hidden module
re-exporting
all generated types.

Also add `CommandPattern` to the pathf pattern analyzer to recognize
`#[command]`-annotated functions and track their generated hidden
modules.

Remove the `origin_return_type` parameter and replace the generic `Into`
conversion with a direct call to `Routable::<#program_type>::to_chain`.
For non-unit returns, also wrap the function call expression in
`to_chain()` upfront to satisfy the `ChainProcess<C>` requirement of
`__modify_res_and_return_route`.