Remove the `pack!`, `pack_err!`, `pack_structural!`, and
`pack_err_structural!` macros, replacing all pipeline type definitions
with `#[derive(Grouped)]` and `#[derive(Grouped, Wrap)]` attributes.
This changes the generated struct shape from named-field structs with an
`inner` field to tuple structs accessed via `.0`, and removes the
auto-generated `name` and `info` fields from error types.

The legacy `parser` feature and its module tree (`mingling::parser`,
`Argument`, `Picker`, `Pickable`, etc.) have been fully removed and
replaced by the `picker` feature powered by `arg-picker`.

BREAKING CHANGE: Remove `parser` feature and use `picker` instead.
Migration guide: Replace `features = ["parser"]` with
`features = ["picker"]` and update API usage per the provided table.

Dispatchers are now always registered at compile time, removing the
`with_dispatcher` / `with_dispatchers` methods and the
`PathfinderConfig` API. The `dispatch_tree` feature now only controls
the matching strategy (trie vs linear list).

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.

std::io::Write import

Replace r_println!/r_print! macro usage across all example renderers
with
explicit RenderResult construction using std::io::Write, enabling more
flexible output handling and reducing reliance on macro-side effects.

`route!()` now calls `Groupped::to_chain()` on the error branch,
removing the need for callers to pre-convert with `.to_chain()` or
`.to_render()`.