Consolidate pre-2026-06-19 release posts into one release-history post (#845)

Every release blog post published before 2026-06-19 (13 posts, from the
first automated release 7.1.20230309 through 10.2.20260618) is folded into
one chronological blog/2026-06-18-release-history.md, preserving each
release's version, date, highlights, and links to the original GitHub
release/changelog. Posts newer than the cutoff, and non-release posts,
are untouched.

- Add 301 redirects in netlify.toml from every retired slug to
  /blog/release-history (no client-redirects plugin is installed, so
  netlify.toml is the only redirect mechanism this repo has).
- tests/release-blog-template.test.js pinned two of the deleted posts
  as format-regression fixtures; move their exact content to
  tests/fixtures/release-posts/ and repoint the test there so it keeps
  validating the release-post generator's historical output shape.
- tests/shaft SiteRenderTest navigated directly to the now-deleted
  /blog/release-10.2.20260501; repoint it at a still-standalone post.

Closes #841


Claude-Session: https://claude.ai/code/session_011YHoYVC5KYCDMmGtwLsEHk

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: MohabMohie