Add `Pickable` implementation for `bool` and the supporting `Matcher`
trait
with utility functions for argument matching.  Expose `pickable_needed`
and
`matcher_needed` modules for downstream trait implementors.

Introduce `or_route` and a `PickerResult` type that carries an optional
route value.  When a flag is absent, the closure given to `or_route` is
evaluated and stored in `error_route`, allowing the caller to exit early
without a default value.

The ParseError variant has been removed and its usages in From,
is_found, is_err, and unwrap have been updated to use NotFound
instead, simplifying the error handling model.

`PickerResult` struct

Introduce `get_attr`, `tag`, and `pick` as the new trait methods,
replacing
the single `tag` method that returned a `PickerTag`.  This enables the
parser to first collect all argument position requirements and then
resolve
them in a second pass.

Rename `PickerTag` to `PickerArgInfo` and reorder `PickerFlagAttr`
variants
to reflect parse priority (`Positional < Flag < Single < Multi`).  Add
`positional_or_else` helper to `PickerFlagAttr`.