Index window of label_count levels that fits in rows, keeping active
visible with the window centered on it and clamped to the ends.
Returns (top, len).
Full-width level gauge mirroring the time bar. label is the current
level’s readout (CF coordinate label or the dimension-name fallback);
index/length position the fill. No-ops for zero-area or single-level
variables so the dashboard can call it unconditionally.