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/__tests__/virtualHeights.test.ts

@@ -31,4 +31,12 @@ describe('virtual height estimates', () => {
       estimatedMsgHeight(msg, 80, { compact: false, details: false })
     )
   })
+
+  it('reserves two extra rows for the inter-turn separator on non-first user messages', () => {
+    const msg: Msg = { role: 'user', text: 'follow-up question' }
+    const base = estimatedMsgHeight(msg, 80, { compact: false, details: false })
+    const withSep = estimatedMsgHeight(msg, 80, { compact: false, details: false, withSeparator: true })
+
+    expect(withSep).toBe(base + 2)
+  })
 })