feat(chat): block-level streaming reveal with a gliding follow

Token-by-token streaming mutates the trailing paragraph in place on
every tick: words rewrap, the last line jitters, and the whole reply
reads as flicker. Streamed text now commits only up to the last complete
line — prose arrives a paragraph at a time (a markdown paragraph is one
logical line), code fences reveal line by line, tables row by row — and
a shown block never changes again. A paragraph that runs long without a
newline releases at the last sentence (then word) boundary so the stream
never stalls. Applies to assistant text and reasoning; tool output keeps
its raw tail.

With growth arriving in block steps, the end follow switches to the
list's animated mode so each step is a glide — reveal and scroll read as
one continuous motion. The gesture opt-out now measures at-end from the
live list state instead of the cached flag, which the animated glide
deliberately leaves stale while trailing the edge; without that, a drag
during a glide could leave the scroll-to-bottom pill unshown.

Measured: end-following holds at distance 0 for the whole stream, and
the mobile drag opt-out shows the pill in three of three runs. The
continuous glide costs ~15% more main-thread time per streamed character
than the instant follow — the price of the motion.

packages/ui/src/components/chat/MessageList.tsx

@@ -1071,7 +1071,10 @@ const TimelineList = React.memo(({
                 // resize settles.
                 maintainScrollAtEnd={anchoredEndSpace || !streamingAutoFollowEnabled || isWidthResizing || endPinningReleased
                     ? false
-                    : { animated: false, on: { dataChange: true, itemLayout: true, layout: true, footerLayout: true } }}
+                    // Animated: with block-level reveal the content grows in
+                    // paragraph/line steps, and the animated follow turns each
+                    // step into a glide — reveal and scroll read as one motion.
+                    : { animated: true, on: { dataChange: true, itemLayout: true, layout: true, footerLayout: true } }}
                 // Prepending older history must not move what the user is
                 // reading. Size restoration applies only during a width
                 // resize — see the observer above.