This introduces Ink.ComboBox, a text field with a pop-up listbox of

auto-complete suggestions, intended to replace the legacy

rbautocomplete plugin and Selectize-based related-object selectors. It

implements the ARIA 1.2 combobox pattern: DOM focus stays on the text

field (an Ink.TextField), the pop-up is a role="listbox", and the

highlighted option is tracked virtually through aria-activedescendant on

the input.
Suggestions can come from a static set of items (via an "items" option

or Ink.ComboBox.Item children, filtered client-side) or from an

asynchronous "load" function suitable for API-backed lookups. Async

queries are debounced, stale responses are discarded, and loading,

empty, and error states are shown in a status area within the pop-up.
Items are backed by a new ComboBoxItem model and

ComboBoxItemsCollection. Options render a label and optional

description by default, with a renderItem hook for fully custom

content (such as avatars).
Three selection modes are supported:


Single selection (the default), with the selected item exposed as

  "value" and serialized to an optional hidden form input via "name".


Multiple selection, showing selected items as removable token pills

  within the field, tracked in a selectedItems collection that can be

  pre-populated or shared. Selected items are excluded from suggestions,

  Backspace on an empty field removes the last token, and the hidden

  input serializes comma-separated item IDs (matching Djblets'

  related-object widgets). The token list is styled like the text field,

  keeping a stable height as tokens are added.


Multiple selection with a consumer-rendered list, where the selection

  is tracked and serialized but rendered by the consumer (such as rows

  with avatars below the field), by listening to events on

selectedItems. This can replace Djblets' Selectize-based

RelatedObjectSelectorView.


There's an optional -is-joined state that allows improving the

  rendering in this mode, joining the search box directly to the list of

  selected results.
Keyboard behavior is designed to feel familiar to users of the legacy

rbautocomplete fields: Arrow keys open the pop-up and move the

highlight with wrapping; PageUp/PageDown jump to the first/last option;

Enter, Tab, and (in multiple mode) comma accept the highlighted option,

with Tab keeping focus in the field so the completion can be seen and

further values typed; Escape closes the pop-up and then clears the

field. Enabling selectFirst reproduces the legacy

tab-completes-the-first-result flow, and a hintText option shows a

hint (such as "Press Tab to auto-complete.") below the results.
In single-selection mode, editing the text away from an accepted item

clears the selection. Otherwise "value" and the hidden input would keep

reporting an item the field no longer shows, and the form would submit

something the user never chose.
Disabling the combo box disables the whole control. In multiple mode

that includes the token remove buttons, which are separate controls from

the field, and the token list draws the disabled state since it holds

the chrome in that mode.
The status strings for the loading, empty, and error states can be

replaced through the "loadingText", "noResultsText", and "errorText"

options, so consumers can pass translated text in.

Ran unit tests covering filtering, keyboard navigation and completion,

  mouse interaction, debouncing, stale-response discarding, error

  states, all three selection modes, serialization, events, selection

  staleness, disabled behavior, and custom status text.
Verified interactively in Chrome and Firefox: typing, Tab/comma

  completion with focus retention, ARIA attribute placement, and stable

  field height with tokens, in both single and multiple modes.
Added storybook entries for static, async, error, tab-completion,

  multiple, and consumer-rendered-list configurations. Verified in both

  light and dark mode.