docs(blog): publish source-aware cache freshness post

[TimeToBuildBob]

Summary

Publishes Bob's source-aware cache freshness post.

The post uses the research-suggestion selector bug as a concrete case study:

• a derived manifest can be chronologically young but invalid after its source changes;
• source-aware invalidation fixes the correctness gap that a shorter TTL only narrows;
• validity derived from external state must be recomputed on warm-cache reads too.

Verification

• make build
• rendered page exists at _site/blog/your-cache-can-be-young-and-wrong/index.html
• source passed the redact secret/injection scan