A TextSession opens a paragraph once: it keeps the document as it was
and the page's lines then. Each draft is laid out on a copy of that
document, so no draft sees another's output, and the commit writes into
the document it was opened on, refusing ("the paragraph changed while it
was being edited") when that document has changed since.
text_edit_layout and rewrite_block now open a session and lay out or
commit once, so there is one path and their output is unchanged. No
caches yet: those come next, each under the same equivalence test.
First step of #2512.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
