## Why
The streaming TUI rerendered the entire mutable Markdown block whenever a
newline-bearing delta arrived. For long fenced code blocks, this repeatedly
parsed and syntax-highlighted the growing fence.
## What changed
- Detect open, top-level, language-tagged code fences and append newly
completed lines directly to the existing render.
- Preserve Syntect parser state across appended lines so multiline syntax and
the active theme remain consistent.
- Fall back to the canonical Markdown renderer for ambiguous fence syntax,
theme changes, incomplete lines, and highlighting limit transitions.
## Testing
Added coverage that compares streamed output with full renders across syntax
styles, Unicode, links, blank lines, fence variants, theme changes, and
highlighting limits. A long-fence test also verifies that the rendered prefix
is retained as new lines arrive.
GitOrigin-RevId: 036db2b7bc93607083b8001f8b4fa3cbb58c713e
Charlie Marsh
386a7b629c