Document Change Assurance Benchmark 0.28.0

A paired static boundary for preview-thumbnail-on-save

DCAB 0.28 adds its thirty-ninth deterministic WordprocessingML case:
review.save_preview_picture_enabled.

The baseline and candidate retain the same package members and stored Word
text. Only word/settings.xml changes from a direct
w:savePreviewPicture/@w:val="false" declaration to true. Neither package
contains a thumbnail relationship or thumbnail image part.

The case is stored-configuration evidence only. Its builder and validator do
not open Word, save a document, create, decode, render, or classify an image,
or make a client-behavior claim. The target-free truth schema is unchanged.

Tagged CI passed on Python 3.11, 3.12, and 3.13; the optional adapter ran
against DocFence 0.38 and achieved the complete 39/39 static-fact score.