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.

Rename the internal fallback type and its associated `ProgramCollect`
plumbing from `ErrorDispatcherNotFound`/`build_dispatcher_not_found` to
`EntryFallback`/`build_entry_fallback`, along with the generated enum
variant and pack type. Update all documentation, examples, and tests
accordingly.

Update all references across docs, examples, and configuration files to
use the shorter `extras` feature name.

fields

Declare `Grouped` as `unsafe trait` to make its soundness invariant
visible.
Make `AnyOutput.type_id` and `member_id` private; add public accessors
and
the `unsafe fn new_bare` constructor.

Remove `Deref<Target=str>` and `render_text:String` in favor of
`render_buffer:Vec<(String, RenderResultMode)>` and an
`immediate_output`
flag. Add `RenderResultMode` enum, `r_eprint!`/`r_eprintln!` macros, and
new buffer manipulation methods (`append_to_buffer`, `eprint`, etc.).
Update `Display` to trim output and remove trailing newline. Migrate all
callers and tests away from `Deref` usage.