Expose `isAtEnd` from `KeyboardChatScrollView` for jump-to-latest and follow-mode UIs

[jkinggg]

Is your feature request related to a problem? Please describe.

KeyboardChatScrollView already computes whether the scroll view is effectively at the end for keyboardLiftBehavior="whenAtEnd", but that state is currently internal.

For chat-style UIs, apps often need the same signal for:

• showing or hiding a "scroll to bottom" / "jump to latest" button
• enabling follow-mode while streaming new assistant content
• tracking whether unread content is accumulating below the viewport

Today the app has to re-implement that logic outside the component. That is awkward because KeyboardChatScrollView already owns the relevant scroll metrics and keyboard-aware padding behavior, so app-side heuristics can drift from the component's internal definition of "at end".

Describe the solution you'd like

Expose an optional isAtEnd?: SharedValue<boolean> prop on KeyboardChatScrollView.

When provided, the component would update that shared value on the UI thread using the same internal scroll metrics and isScrollAtEnd(...) logic it already uses for keyboardLiftBehavior="whenAtEnd".

Example:

const isAtEnd = useSharedValue(true);

<KeyboardChatScrollView
  isAtEnd={isAtEnd}
  keyboardLiftBehavior="whenAtEnd"
/>

This keeps the public API narrow while still exposing the high-level state chat apps actually need.

Describe alternatives you've considered

1. Expose raw scroll position instead.

I think that is a less ideal API for this use case. Most consumers do not need raw scrollY; they need the semantic "is the user pinned to the latest content?" state. Exposing raw scroll metrics also leaks more implementation detail and pushes more edge-case handling into app code.

2. Recompute "at end" in the app.

This works imperfectly, especially once keyboard lift behavior, bottom padding, composer growth, and inverted/non-inverted behavior are involved. The component already has the authoritative inputs, so surfacing that result is more reliable.

3. Expose a callback instead of a shared value.

A callback is usable, but a SharedValue<boolean> fits the existing Reanimated-driven API surface better and avoids bouncing this state back to JS for consumers that want to animate directly from it.

Additional context

• This would be an additive API change.
• The implementation can be JS-only in KeyboardChatScrollView.
• Internally, the component already has everything needed: scroll, layout, size, and isScrollAtEnd(...).
• This is especially helpful for AI chat and messenger-style screens that need a jump-to-latest button that stays in sync with the component's own whenAtEnd behavior.