feat(document): split, merge and insert a paragraph

[andiwand]

🤖 Generated with Claude Code

Third of five; #873 is merged, so this now sits on main. See
docs/design/document-editing.md.

What this one does

• splitParagraph moves what follows the element it names into a new paragraph
  carrying the same style.
• mergeParagraph takes the children of the paragraph after this one and
  removes it.
• insertParagraph puts an empty one after it.

With the run operations, Enter, Backspace at the start of a paragraph
and a delete spanning paragraphs are all replayable.
Document::split_paragraph, merge_paragraphs and insert_paragraph_after
are the same three in the C++ API — on the document rather than on a handle,
for the reason the run operations are: a handle says what an element holds,
the document what the tree holds. Each refuses an element of another document.

A split names a descendant, not a direct child

The caret sits in a run and the run sits in a span, so the split walks from
that run up to the paragraph and splits every element on the way: a run
inside a span leaves the span in both halves, and the tail keeps the formatting
it had. The same holds for a link, so the tail is still a link to the same
place.

Only a span and a link are split through. Anything else — a frame
between the run and the paragraph — refuses with UnsupportedOperation,
because what a copy of it would mean is the format's question rather than this
one's.

A copy carries the original's attributes and the property children the format
writes ahead of the content — w:pPr on a paragraph, w:rPr on a run. Those
sit before the first child the registry knows about, which is how the copy
finds them without naming a tag. ODF states the same thing as an attribute, so
one rule covers both.

Verified

Headless LibreOffice reads the split paragraph out of both a saved .odt and a
saved .docx as two paragraphs, the second keeping the style of the first.