locallibrary: a chart on the sidecar diagrams page sometimes fails to draw on GitHub #482

docs/diagrams/sidecar.md has 10 charts. In one of five loads on GitHub, one of them showed "Unable to render rich display" where the chart should be. The other four loads drew all 10, and a reload clears it. The load that failed didn't record which chart it was, so the cause isn't known.

GitHub's limit on charts per page is what tools/readmes guards, at 40, and this page is well under it. The large method maps on this page may be slower to draw, but that hasn't been shown.

How to test: load the page repeatedly and count the visible error boxes inside section[data-type="mermaid"], noting which chart each one is in. If one chart is the cause, split it and repeat.