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/lib/virtualHeights.ts

@@ -43,8 +43,15 @@ export const estimatedMsgHeight = (
     compact,
     details,
     limitHistory = false,
-    userPrompt = ''
-  }: { compact: boolean; details: boolean; limitHistory?: boolean; userPrompt?: string }
+    userPrompt = '',
+    withSeparator = false
+  }: {
+    compact: boolean
+    details: boolean
+    limitHistory?: boolean
+    userPrompt?: string
+    withSeparator?: boolean
+  }
 ) => {
   if (msg.kind === 'intro') {
     return msg.info?.version ? 9 : 5
@@ -80,5 +87,12 @@ export const estimatedMsgHeight = (
     h++
   }
 
+  // Inter-turn separator above non-first user messages (1 rule row + 1
+  // top-margin row). The render-side gate is in appLayout.tsx; we trust
+  // the caller to pass `withSeparator` only when it matches that gate.
+  if (withSeparator) {
+    h += 2
+  }
+
   return Math.max(1, h)
 }