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Keeps a marquee clone's interactive elements clickable while removing them from keyboard tab order and the accessibility tree — solving the "clone cards swallow clicks" bug caused by using `inert`.

## Key Components

**`CLONE_FOCUSABLE_SELECTOR`** — Comprehensive CSS selector covering all natively focusable elements: anchors, form controls, `iframe`, `summary`, media with `controls`, `[tabindex]`, and editable content regions.

**`useSuppressCloneFocus(active: boolean)`** — React hook that returns a `ref` to attach to a clone wrapper `<div>`. When `active`, it:
- Sets `tabindex="-1"` on every focusable descendant (removes duplicate tab stops)
- Watches for DOM mutations that could re-introduce tabbable elements via `MutationObserver`
- Leaves click/pointer behavior fully intact (unlike `inert`, which blocks all interaction)
- Re-applies suppression only when `tabindex` hasn't already been stamped, preventing infinite observer loops

## Usage Example

```typescript
import { useSuppressCloneFocus } from './use-suppress-clone-focus'

function MarqueeClone({ isClone }: { isClone: boolean }) {
  const cloneRef = useSuppressCloneFocus(isClone)

  return (
    // aria-hidden keeps clone out of a11y tree;
    // hook keeps it out of tab order while staying clickable
    <div ref={cloneRef} aria-hidden={isClone}>
      <a href="/article-1">Card One</a>
      <a href="/article-2">Card Two</a>
    </div>
  )
}
```

## Accessibility Contract

| Mechanism | Handled by |
|---|---|
| Out of a11y tree | `aria-hidden` on clone wrapper (caller's responsibility) |
| Out of tab order | This hook (`tabindex="-1"` on all focusable descendants) |
| Clickable / pointer events | Preserved — clone anchors handle mouse, <kbd>⌘</kbd>-click, middle-click, and context menu natively |

> **Why not `inert`?** The `inert` attribute also disables pointer events, causing clone cards near the seam of an endless-loop scroll to silently swallow all clicks — the core bug this hook is designed to prevent.