docs: fix broken star history chart in README

[FaintFlower]

The README star history chart is currently broken due to the GitHub stargazer API restrictions. This switches the chart to a working alternative at star-history.dera.page, restoring the visual so the project's growth is displayed correctly again.

[1716775457damn]

Good fix — the GitHub stargazer API limitation does break the chart silently. Swapping to the dera.page alternative restores the visual without maintaining our own data pipeline. One thing worth verifying: the alternative service may have some cache delay before it picks up new stars, so the chart could briefly lag for a few days after this merges. That's an acceptable trade-off for a stable README badge.

[1716775457damn]

Reasonable fix, and the diagnosis matches: the stargazers endpoint requires auth and is aggressively rate-limited, which is why these charts tend to fail intermittently rather than hard-fail — pinning to a renderer that handles that is the pragmatic call.

One thing worth considering for the longer term: an externally-hosted SVG turns every README render into a third-party request, so the chart breaks again whenever that host has an outage or rotates its URL scheme, and it exposes viewer IPs to it. Generating the chart on a schedule in a workflow and committing the image into the repo (or rendering via gh chart) keeps it on GitHub's CDN behind a relative path — no runtime dependency, and no re-break the next time a community instance changes hands.

Not a blocker for this PR, just worth tracking if it breaks a second time.