Render streamed Markdown incrementally (#34045)

## Why

Streaming TUI responses repeatedly rendered the entire accumulated Markdown source for each committed delta, including blocks whose output could no longer change.

## What changed

- Retain rendered output for completed top-level Markdown blocks and rerender only the final mutable block as new source arrives.
- Track block boundaries during the existing Markdown parser pass and map them through table-fence normalization.
- Preserve canonical full rendering when source-wide state may affect earlier output, including reference link definitions and inline visualizations, and when the width or render mode changes.
- Append raw-mode lines directly while preserving blank lines.

## Testing

Added coverage that compares incremental output with full rendering across paragraphs, headings, lists, block quotes, code fences, tables, HTML blocks, links, visualizations, render-mode changes, and normalized table fences.

GitOrigin-RevId: 748171c4ed58923329b59c48ea3c2afa4819ec88

codex-rs/tui/src/markdown_render.rs

@@ -77,8 +77,12 @@ use unicode_width::UnicodeWidthChar;
 use unicode_width::UnicodeWidthStr;
 use url::Url;
 
+mod streaming;
 mod table_key_value;
 
+pub(crate) use streaming::StreamingMarkdownRender;
+pub(crate) use streaming::render_streaming_markdown_lines_with_width_and_cwd;
+
 const TABLE_COLUMN_GAP: usize = 2;
 const TABLE_CELL_PADDING: usize = 1;
 const TABLE_HEADER_SEPARATOR_CHAR: char = '━';