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/message/parts/assistantTextVisibility.ts

@@ -1,9 +1,16 @@
+import { commitStreamedText } from '../../lib/streamTextCommit';
+
 export const resolveAssistantDisplayText = (input: {
     textContent: string;
     throttledTextContent: string;
     isStreaming: boolean;
 }): string => {
-    return input.isStreaming ? input.throttledTextContent : input.textContent;
+    // While streaming, reveal whole blocks only: rendering stops at the last
+    // complete line so a shown paragraph never mutates in place. The held
+    // tail lands with the next line break (or the finalize pass).
+    return input.isStreaming
+        ? commitStreamedText(input.throttledTextContent)
+        : input.textContent;
 };
 
 export const shouldRenderAssistantText = (input: {