…tion ↻cache)

The status bar showed session-aggregate cache-hit % and the
running turn's wall-clock + token count, but the actual per-turn
prompt/completion/cache numbers were only visible by digging
into the WireUsage payload in devtools. For a cost-aware user
(especially on DeepSeek where cache hits are 90% cheaper than
misses) the billable numbers deserve a permanent place.

Add components/UsageIndicator.tsx: a 3-cell pill that mounts in
the status bar to the right of the cache-hit %. Each cell is a
tiny caption + value pair:

  ↑ 1.2k   prompt tokens this turn
  ↓  280   completion tokens this turn
  ↻  850   cache-hit tokens (only rendered when > 0)

The values are k-rounded (1.2k / 18k / 0) to keep the bar
compact. The cache cell uses the existing --add-fg token so the
green cue matches the rest of the UI.

A real sparkline (per-turn history) needs a separate ring buffer
in the controller — the WireUsage payload only carries the latest
turn. That's a follow-up; the pill is the immediate win and the
sparkline is a drop-in upgrade.

StatusBar.tsx mounts the pill when usage is present (the
controller dispatches a usage event on every model step, so the
pill ticks as the turn progresses).