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/inline_visualization.rs

@@ -24,7 +24,7 @@ use uuid::Uuid;
 
 use self::viewer::materialize_document;
 
-const DIRECTIVE_PREFIX: &str = "::codex-inline-vis{";
+pub(crate) const DIRECTIVE_PREFIX: &str = "::codex-inline-vis{";
 const MAX_FRAGMENT_BYTES: u64 = 2 * 1024 * 1024;
 
 #[derive(Clone, Debug)]