Update changelog and issue tracker notes for the enhanced
`default_completion` behavior.

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.

Add `#[completion(EntryFallback)]` syntax to enable custom
completion suggestions for unmatched entries, merging fallback
results with default completions via `Suggest::combine()`.

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.

Add convenience presets (`mini`, `advanced`, `full`, `build_advanced`,
`build_full`) to `mingling/Cargo.toml` and document them in both
English and Chinese feature docs. Update getting-started guides to
reflect the new default `core`/`macros` features.

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