FE-681: add multiselect to ds

[alex-e-leon]

🌟 What is the purpose of this PR?

• Adds multiple prop to select to enable multi select
• Adds searcheable prop to select

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

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[cursor]

PR Summary

Medium Risk
Broad API and behavior changes to a core form Select (value shapes, Ark multi mode, keyboard/search edge cases); existing single-select callers should be unaffected but regressions in focus, filtering, or selection are possible.

Overview
Adds multi-select to Select via multiple: true, with array value/onChange, optional maxItems, per-item MultiSelectItem options (checkbox/tick/highlight indicators, tones, suffixes, hover "Only" to isolate one value), and the dropdown staying open while selecting.

Adds optional searchable filtering (label text, multi-term) with onSearch, empty states, and SearchHighlightSync so keyboard navigation from the search field targets visible matches. Search UI is shared as SelectableListSearch, also adopted by SortMenu; list layout/CSS moves the search row when the popover flips upward, and SelectableList passes arrow/Enter through custom rows in Select mode.

Single-select behavior is preserved with stricter typings (SelectSingleProps vs SelectMultipleProps). Minor UX fixes: clear-button visibility for keyboard focus and multi-select list row spacing. MultiSelectItem is exported; Ladle stories cover multi-select and single-select searchable.

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