When idle with a top-level delegate_task still in flight, render a static,
shimmering system-note at the transcript tail instead of a spinner (which
reads as "stuck"). Reuses the shared steer / slash-status chrome (centered,
0.6875rem, muted, Codicon) so it sits in the thread like every other meta
line, and mirrors the primary child's latest stream line, falling back to
generic copy. i18n across en/ja/zh/zh-hant; markdown prose/heading rhythm
tuned so a re-entered turn breathes.