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/appChrome.tsx

@@ -23,9 +23,7 @@ const HEART_COLORS = ['#ff5fa2', '#ff4d6d']
 // Keep verb segment width stable so status-bar content to the right doesn't
 // jitter when the ticker rotates between short/long verbs.
 export const VERB_PAD_LEN = VERBS.reduce((max, v) => Math.max(max, v.length), 0) + 1 // + ellipsis
-export const DURATION_PAD_LEN = 7 // e.g. "  9s", "1m 05s", "59m 59s"
 export const padVerb = (verb: string) => `${verb}…`.padEnd(VERB_PAD_LEN, ' ')
-export const padTickerDuration = (ms: number) => fmtDuration(ms).padStart(DURATION_PAD_LEN, ' ')
 
 // Compact alternates for the `emoji` and `ascii` indicator styles.
 // Each entry is a fixed-width (display-width) glyph.
@@ -114,7 +112,7 @@ function FaceTicker({ color, startedAt }: { color: string; startedAt?: null | nu
   // verb segment is hidden (e.g. `unicode` spinner style).  When the verb
   // IS shown, its trailing padding already provides the gap, so the extra
   // space is harmless.
-  const durationSegment = startedAt ? ` · ${padTickerDuration(now - startedAt)}` : ''
+  const durationSegment = startedAt ? ` · ${fmtDuration(now - startedAt)}` : ''
 
   return (
     <Text color={color}>

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} />