refactor(pickable): redesign Pickable trait with two-phase parse API

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`.

0 files changed, 0 insertions, 0 deletions