Add a mutation driver over the page model (text replacement, character
formatting, paragraph insertion at nesting boundaries, subtree deletion and
reordering, collapse, outline geometry and outline insertion) that publishes
each mutated model through the page writer and checks that the written page
reads back as the model, that an unchanged model publishes nothing, and that
objects outside the model keep their bytes. Rejections are allowed only as
explicit writer errors. A deterministic test runs four seeds and 400
generated inputs; the page_model fuzz target runs the same driver.

Assisted-by: claude-fable-5.1