Unverifiable citation in How I Read, Eight Years On: Medium article for the spacing effect

## Summary

`src/data/blog/how-i-read-eight-years-on.md:90` cites the spacing effect through Medium:

```
[spacing]: https://onyxboox.medium.com/micro-learning-made-easy-learn-something-new-with-boox-192f481349be
```

Medium refuses every automated request, so nothing can confirm the article is there. The refusal is not specific to this URL — a slug invented on the spot gets the identical answer:

```
403 boox-back-to-school-promo-upgrade-your-learning-device-at-new-beginnings-2c88054619da
403 4-workflow-optimization-strategies-with-boox-da9d21d08af3
403 micro-learning-made-easy-learn-something-new-with-boox-192f481349be
403 this-article-does-not-exist-000000000000
```

## Why it may actually be gone

The same article is published at `https://shop.boox.com/blogs/news/micro-learning-made-easy-learn-something-new-with-boox`, which serves 200 and mentions the spacing effect three times — matching what the post cites it for. A search for the title surfaces that copy and a LinkedIn version, but not a Medium one, and the `onyxboox.medium.com` feed doesn't list it. Onyx has been moving off its older domains: `onyxboox.com` no longer answers at all (#488).

Repointing `[spacing]` at the shop.boox.com copy would make the citation checkable and drop it out of the weekly report. Worth confirming the two are the same piece first.

## Context

Left out of `lychee.toml`'s `exclude` deliberately in #483: an exclusion asserts a link is good, and there's no evidence for that here. It will appear in the weekly link report until this is resolved.