Renders a multi-value select (html_type: :select with data.select.multiple) as a list of checkboxes over its options.

This is the scalar counterpart of Aurora.Uix.Templates.Basic.Renderers.ManyToMany, and it exists for the same reason: a <select multiple> needs a modifier-key gesture that is undiscoverable and unusable on touch, conveys state only through a background colour, and has nowhere to host bulk controls. The wire format is identical -- every box shares the parent[field][] name, so only checked values are submitted, exactly as only selected options are.

Options come straight from the parsed field (data.select.opts) through Aurora.Uix.Templates.Basic.Helpers.get_select_options/1, so both backends reach this renderer unchanged: Ctx parses {:array, Ecto.Enum} and Ash parses an array attribute whose item type carries a one_of constraint (or is an Ash.Type.Enum module) into the same shape.

Key Features

• :form renders one checkbox per option; the checked boxes are the current value.
• Submits in the same POST as the parent, under parent[field][].
• Emits a hidden empty-value sentinel so that de-selecting everything still submits the key, which is what makes clearing the last value possible at all.
• Ships :default_toggle_all, a tri-state checkbox beside the 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 three action groups through Aurora.Uix.Action -- label (holding the toggle), header and footer, the latter two empty -- so a host adds, replaces or removes controls in any of the three strips from the layout DSL field options.
• :show renders only the selected options as a plain, read-only list, with the dt("No options to show") empty state.

Key Constraints

• The library is transport-only for writes: it renders the input name and forwards the submitted list untouched, and never builds a changeset.
• Because of the sentinel, the submitted list always carries one blank entry. The host must reject it -- an Ecto.Enum array otherwise fails to cast, and Ash rejects the list with "no nil values" unless the attribute or argument declares constraints: [nil_items?: true].
• The toggle rides the parent form's phx-change="validate", identified by _target. 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.
• Hidden fields never reach this renderer -- DefaultRenderer keeps them on its generic clause, which renders them as a single <input type="hidden">.