- add useStreamingMarkdown: cap mid-stream markdown re-render to ~140ms,
full-fidelity render once the segment completes
- skip code-block language auto-detection while streaming (escaped plain
text), restore full highlighting on the final render
- defer echarts/mermaid blocks to a lightweight loading placeholder while
streaming so their parsers never run on truncated source
- bypass the render cache for streaming-mode output
- wire into ContentSegment and MessageBubble (content + thinking)
