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React context that bridges `useDocumentTree` viewer instances to `GlobalAskAI` components, enabling programmatic document navigation without global events or URL parsing.

## Key Components

### `DocNavigator` (interface)
Represents a mounted document viewer with three members:
- `baseRoute` — the URL prefix this navigator owns (e.g., `'/knowledge-base'`)
- `findNodeByPath(path)` — looks up a `DocNode` by storage path
- `selectNode(node)` — triggers navigation as if the user clicked a sidebar item

### `DocNavigationProvider`
Context provider that maintains a `Map<baseRoute, DocNavigator>`. Supports multiple concurrent navigators (dual-pane, nested viewers) with longest-prefix matching to route navigation calls to the correct owner.

### `useDocNavigation()` (hook)
Returns the context value with three methods:
- `register(nav)` — registers a navigator; returns a StrictMode-safe cleanup function
- `navigate(path)` — resolves the owning navigator and selects the node; returns `true` if handled
- `isAvailable()` — returns `true` if at least one navigator is mounted

## Usage Example

```typescript
// In a document viewer component
import { useDocNavigation } from './doc-navigation-context'

function DocViewer({ baseRoute }: { baseRoute: string }) {
  const { register } = useDocNavigation()

  useEffect(() => {
    const cleanup = register({
      baseRoute,
      findNodeByPath: (path) => tree.find(path) ?? null,
      selectNode: (node) => setActiveNode(node),
    })
    return cleanup
  }, [baseRoute, register])
}

// In GlobalAskAI — navigate to a cited document
function AskAI() {
  const { navigate, isAvailable } = useDocNavigation()

  const handleCitationClick = (storagePath: string) => {
    const handled = navigate(storagePath)
    if (!handled) window.open(storagePath, '_blank')
  }
}
```