Highlighting reuses the existing code-block tokenizer and LRU cache, runs
lazily on expand, and falls back to plain text above a size cap so a large
Read can't stall the main thread. The diff highlighter reconstructs the old
and new file text by position rather than from @@ line ranges, so diffs that
arrive without real hunk headers still highlight.
Read content is normalized server-side: the cat -n line-number gutter some
providers emit is stripped so content is uniformly raw source, with the first
line number surfaced as offset for the client to rebuild the gutter.
