feat(tui): segment turns with rule above non-first user msgs; trim ticker dead space (#21846)

Multi-turn transcripts ran together visually because every user message
got the same vertical rhythm regardless of position. Adds a short ─── in
the border colour above every user message after the first, so each turn
reads as its own block. Height estimator gains a `withSeparator` flag so
virtual scrolling pre-allocates the extra two rows (rule + top margin)
and avoids a jump on first measurement.

While in the area: the busy-indicator duration was padded with
`padStart(7)`, leaving five visible spaces between `·` and the digits
(`⠋ ·      2s`) — especially loud under the verb-less `unicode` style.
Drop the padding entirely (`⠋ · 2s`); the model label now shifts a few
columns as the duration grows, which is the right trade-off for the
minimal indicator styles. The verb-padding test stays; the
duration-padding test is removed alongside the function it covered.

ui-tui/src/components/appLayout.tsx

@@ -76,6 +76,15 @@ const TranscriptPane = memo(function TranscriptPane({
     return -1
   }, [transcript.historyItems])
 
+  // Index of the first user-role message; every later user message gets a
+  // small dash above it so multi-turn transcripts visually segment by
+  // turn. -1 when no user message has been sent yet → no separator ever
+  // renders.
+  const firstUserIdx = useMemo(
+    () => transcript.historyItems.findIndex(m => m.role === 'user'),
+    [transcript.historyItems]
+  )
+
   return (
     <>
       <ScrollBox
@@ -95,6 +104,12 @@ const TranscriptPane = memo(function TranscriptPane({
 
           {transcript.virtualRows.slice(transcript.virtualHistory.start, transcript.virtualHistory.end).map(row => (
             <Box flexDirection="column" key={row.key} ref={transcript.virtualHistory.measureRef(row.key)}>
+              {row.msg.role === 'user' && firstUserIdx >= 0 && row.index > firstUserIdx && (
+                <Box marginTop={1}>
+                  <Text color={ui.theme.color.border}>───</Text>
+                </Box>
+              )}
+
               {row.msg.kind === 'intro' ? (
                 <Box flexDirection="column" paddingTop={1}>
                   <Banner t={ui.theme} />