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__/statusBarTicker.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 
-import { DURATION_PAD_LEN, padTickerDuration, padVerb, VERB_PAD_LEN } from '../components/appChrome.js'
+import { padVerb, VERB_PAD_LEN } from '../components/appChrome.js'
 import { VERBS } from '../content/verbs.js'
 
 describe('FaceTicker verb padding', () => {
@@ -16,12 +16,3 @@ describe('FaceTicker verb padding', () => {
     }
   })
 })
-
-describe('FaceTicker duration padding', () => {
-  it('keeps elapsed segment width stable across second/minute boundaries', () => {
-    const samples = [9000, 10000, 59000, 60000, 61000, 3599000]
-    const lens = samples.map(ms => padTickerDuration(ms).length)
-
-    expect(new Set(lens)).toEqual(new Set([DURATION_PAD_LEN]))
-  })
-})

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)
+  })
 })