Canonical visible model for one buffer-like window.

The Content stage builds this model from current-frame data. Frontend adapters encode it for GUI or composite it into cells for TUI proof-of-concept paths. The struct is pure data and lives in core so products can produce window content without importing MingaEditor.

contiguous_rows is a BEAM-internal hint (never encoded on the wire): it is true only for the non-wrapped, non-folded sequential path, where rows are consecutive :normal buffer lines. It lets ScrollPresentation derive the resident line range by arithmetic instead of folding over every row.

content_digest is a BEAM-internal, never-encoded incremental fingerprint of the row set (Minga.RenderModel.Window.ContentDigest), set only on the full-document residence path. When present, the GUI adapter's content frame-emit gate uses it instead of hashing the whole rows list, so an edit-frame gate is O(changed rows) rather than O(document). It is nil off the residence path, where the adapter keeps hashing rows directly.

scroll_seq is the monotonic scroll-authority sequence (#2661) encoded onto ScrollPresentation. It advances when the committed viewport top changes for a reason other than an echoed frontend scroll report, or when an authoritative viewport-jump command explicitly marked the window even though the top was unchanged (#2652); see MingaEditor.Window.settle_scroll_seq/1. Frontends discard their local offset on any increase, so a BEAM-initiated jump racing a local scroll is distinguished from the frontend's own reported delta being reflected back.