spec-by6.4 A code fence whose only change is its info string renders as unchanged Past Stand

prosediff.Block.Lines excludes the fence delimiters, so a change from ```go to ```python produces no line-script difference. Stats counts the block as modified (BlocksModified: 1) while the unified diff table renders every one of its lines as ph-r-eq context. The page therefore says 'this document changed' and then shows nothing that changed.

Found by adversarial verification of spec-by6.3.5; pre-existing in the segmenter, not introduced by the line-numbered diff port. It matters more now than it did under the block-card UI, because the table's whole contract is that a row states what happened to that line.

Fix is in prosediff: either carry the info string into the block's diffed content, or have the web layer render an explicit marker row when Block.Info differs across a ChangeModify.