TypeScript: have items and selectedItems as Readonly

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**Relevant code or config**

```javascript
```

**What you did**:

**What happened**:

**Reproduction repository**:

**Problem description**:
Since we use items/selectedItems just for some logic and we do not tamper with them, we can mark them as read only in the TS interface.

**Suggested solution**:
In typings.ts for useSelect, useCombobox and useMultipleSelection, use Readonly to wrap the items type.

Start in typings.ts and inspect the type definitions for useSelect, useCombobox, and useMultipleSelection. Confirm how items and selectedItems are currently typed, then update those exposed types to use Readonly as requested. Done means all three hook interfaces prevent mutation through their TypeScript types while retaining the existing item shapes.

Written by the indexing model from the issue text.