Render a finished assistant reply as one widget.
Textual's Markdown mounts one child widget per block, list item, bullet,
table cell, and fence label. A long transcript therefore holds thousands of
widgets, and every relayout walks all of them. A reply that no longer streams
does not need that structure, so FinalMarkdown lays out the same markdown
as rows of Content inside a single widget.
The layout copies the block styles, margins, and wrapping of Textual's
Markdown so a reply keeps its height when the streamed Markdown is
swapped for this widget. Validated against Textual 8.2.8: re-check
MarkdownBlock._token_to_content, the Markdown block CSS, GridLayout
table sizing, and textual._resolve.resolve on every Textual bump.
