- MessageList rewritten on vue-virtual-scroller DynamicScroller; only rows
  near the viewport mount (session switch 650-730ms longtask / DOM 31645 ->
  110-165ms / DOM 1-2.5k, zero freezes)
- scroller is keyed by session id: message ids repeat across sessions, so
  teardown drops the measured-size cache on switch
- convergeToBottom watches the bottom hold for a bounded window after
  landing (lazy row measurements nudge scrollHeight late); yields to wheel/
  pointer/touch input instead of scroll-position heuristics, which the
  scroller's own adjustments would trip
- renderMarkdown gets a byte-capped LRU memo so markdown remount cost is
  amortized while virtualized rows recycle
- removed dead .message-list/.message-list-inner styles