[samsoir]

Closes #355

What

The Prefetch System pane's meter row reads Utilization instead of Capacity, with its indent dropped so its values align with the Prefetch row above. Beside the meter a sparkline shows the lane's utilization over the last twelve seconds.

   Prefetch: ● Active │ Cruise │ 42/cyc
Utilization: █████████░  93% │ ▁▁▂▅▇████▇▅▃ │ 1,904 queued

Two choices a reviewer cannot read off the diff:

• One cell per second, not per draw. The other panes push a sample every draw, and the dashboard draws every 100 ms, so their 12-cell sparklines cover 1.2 s. LaneHistory averages each second into a cell and draws the open second live, so the row covers 12 s and still moves on the next draw.
• Scaled against a full lane, not the recent peak. Utilization has a ceiling the throughput sparklines lack, so a half-busy lane always draws at half height and agrees with the percentage beside it.

Verification

• make pre-commit clean: fmt, clippy with -D warnings, full workspace tests.
• 11 widget tests (6 new) cover the label, the alignment with the Prefetch row, the sparkline position in the row, per-second averaging, absolute scaling, the 12-second window and an unknown lane size reading as idle.
• Not yet seen in a running dashboard.

CHANGELOG entry under Unreleased.