fix: add opt-in scroll restoration for view transitions

[sauravhathi]

Problem

When <ViewTransitions> wraps a Next.js app, scroll restoration stops working. Navigating to a page and pressing back always scrolls to the top.

Root cause: The popstate handler in browser-native-events.ts wraps navigation in document.startViewTransition(), which defers DOM updates and prevents the browser's/Next.js's scroll restoration from working.

Solution

Adds opt-in scroll restoration via the options prop:

<ViewTransitions options={{ scroll: true }}>
  {children}
</ViewTransitions>

When enabled:

1. Scroll positions are persisted into history.state via replaceState (debounced at 100ms)
2. On popstate, the saved position is read from the target history entry
3. Scroll is restored before the view transition resolves, so the new DOM snapshot captures the correct position

This follows the same pattern used by Remix and React Router — positions are stored in history.state.

Changes

• browser-native-events.ts: Added saveScrollPosition helper, debounced scroll listener, and scroll restore before transition resolve
• transition-context.tsx: Added ViewTransitionsOptions type and options prop to ViewTransitions component

Breaking Changes

None. Scroll restoration is opt-in and disabled by default.

Fixes #58
Fixes #12

[vercel]

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