Function render_web_fragment 

pub fn render_web_fragment(tagged: &[TaggedRun]) -> String

Renders an HTML fragment with no <html> wrapper, for the Zola website to embed.

The fragment holds the averaged scalar summary, and then throughput, CPU, and memory over the run window for each scenario. Every individual run is a faint line, and the across-run mean is a bold line. tagged pairs each run with the runNN tag parsed from the result filename, so every per-run trace carries a name.