persistThumbnailMetadata was a four-arg method signature that mixed
three conceptually related values. Wrapping them in a private
ThumbnailResult record drops the signature to (Document, result),
mirrors the existing SourcePreview record one step earlier in the
pipeline, and keeps generate() reading as a narrative of small
named outputs rather than positional arguments.
Refs #305
Fixes @felixbrandt suggestion 2 from PR review
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
78caac8d1a