Content: the canonical content model for Quillmark.
One [model::Content] per content field: a single text sequence carrying
line attributes, anchored marks, and embedded islands, over one coordinate
space of Unicode scalar values. Markdown is a projection
([import::from_markdown], [export::to_markdown]), so every edit is a
splice and all structure moves with it. [serial] is the canonical,
byte-deterministic JSON that storage, the render seam and the binding seam
all carry; [delta] and [ops] are the per-field edit surface.