fix(chat): rebuild auto-follow on a single instant writer to kill scroll twitch

Replace the RAF easing follow loop + settle burst with an always-on
instant-follow model: while pinned, the content ResizeObserver re-pins to
the bottom synchronously (scrollTop = scrollHeight, before paint) and is
the only writer of scrollTop. A position+TTL auto marker distinguishes our
own programmatic writes from genuine user scrolling, so a scroll event that
lands at the just-written bottom never trips a false release.

This removes the feedback loop where the easing animation, growing content,
and the user's own scroll all fought for scrollTop in the same frame -- the
infinite twitch when scrolling down during streaming, and the jiggle on
send / from the queue. The public hook interface is unchanged; all
consumers keep working untouched.

packages/ui/src/components/chat/hooks/useChatTimelineController.ts

@@ -382,12 +382,13 @@ export const useChatTimelineController = ({
             // through goToBottom — the single programmatic writer.
             //
             // A normal bottom APPEND (a sent message, a streaming part) must NOT
-            // re-pin here. Auto-follow's own follow loop — kicked by the content
-            // ResizeObserver and the streaming chunk handlers — already eases to the
-            // new bottom. Calling goToBottom on every append layered its settle burst
-            // on top of that loop: two writers aiming at different positions, which
-            // is exactly the up/down jiggle reported on send / from the queue / while
-            // streaming. So for an append we do nothing and let the follow loop own it.
+            // re-pin here. Auto-follow already owns the bottom: its content
+            // ResizeObserver re-pins instantly (scrollTop = scrollHeight, before
+            // paint) on every append. Re-pinning again from here would just be a
+            // second writer chasing the same target a frame later — redundant at
+            // best, and the source of the old up/down jiggle on send / from the
+            // queue / while streaming. So for an append we do nothing and let
+            // auto-follow own it.
             if (snap || isPrepend) {
                 prePrependScrollRef.current = null;
                 goToBottom('instant');