Add historical graph generation workflow

[perpil]

Pull Request Checklist

For New Runtime Additions

• Added the new runtime to the manifest.json file
• Updated README.md to reflect the new runtime
• Gave execution permission to build.sh
• If needed, updated .github/dependabot.yml
• If needed, updated .gitignore

Description

Add graphs to show historical trends over time to the website. This is useful in visualizing improvements and regressions in runtimes.

• Add a GitHub Actions workflow to generate and commit SVG performance graphs when data changes.
• Introduce graph-generator/app.py to parse data JSONs, produce 2y and 180d SVG charts and commit them to /graphs.
• Inject a small clickable graph thumbnail into index.html, add CSS for graph container and thumbnail styling, and update script.js to set thumbnail and large-graph URLs (handles GitHub Pages vs local).
• Uses pandas/matplotlib to generate graphs
• Gemini CLI w/model 3.0 Flash was used to create this commit

Fixes #3609

Appearance

If you click into one of the graphs it looks like this:

Author Concerns

1. I have only run this locally, I'm not familiar enough with github workflows to test that the script runs in the github environment.
2. The files it generates go into the graphs folder and are committed to the repro. Since they are SVGs, they should be relatively small diffs, but there are 1400 of them per day that overwrite the same file.
3. This will load 35 svg files from the github raw endpoint on page load. It is similar to how data.json is loaded, but this might not scale as well as having the files not checked into the repo and pushed to the CDN with the index.html.

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