Provides helpers for managing multi-value select actions in Aurora UIX form layouts.

This module sets up and modifies actions for the three strips around the checkbox list that represents a multi-value select: beside the label, at the top right, and below the list. It is the scalar counterpart of Aurora.Uix.Templates.Basic.Actions.ManyToMany, which does the same for a many-to-many membership.

Key Features

• Adds :default_toggle_all, a tri-state checkbox rendered beside the group label: checked when every option is selected, unchecked when none is, and a dash when only some are. Clicking a checked toggle clears the selection; clicking it in either other state selects everything.
• Registers empty header and footer groups, so a host always has an extension point above and below the checkbox list even though the library ships no default there.
• Integrates with the Aurora UIX action modification pipeline, so hosts add, insert, replace or remove actions from the layout DSL field options.

Key Constraints

• Actions render only for layout_type: :form; every component falls back to ~H"" otherwise.
• The toggle rides the parent form's phx-change="validate" and is identified by _target (see Aurora.Uix.Templates.Basic.Handlers.FormImpl). It must not use phx-click: a click on a checkbox fires click and then change, and the trailing change would re-validate with the pre-click selection and undo the toggle.
• Its input name is deliberately outside the resource scope, so the toggle never reaches the host's changeset or Ash action as an unknown key. It shares the auix_toggle_all__ prefix with the many-to-many toggle because both are resolved by the same FormImpl clause.
• Expects the assigns map to include an :auix key with :layout_type, :multi_select_toggle_state and the layout tree options, plus the :field being rendered.