v0.24.2: test: pin that the cached pristine probe is never shared into an entry

`_pristine_frontmatter` is `@cache`d, so a single dict backs every entry
of a type. The write path only ever compares against it, but if one of
its values were carried into an entry's frontmatter, a mutation anywhere
would corrupt every later save of that type -- silently, and for every
entry at once. Cheap to pin, expensive to discover.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>