fix(markdown-stream): incremental rendering to prevent browser freezing

[JamesHWade]

Fixes #164

Summary

Fixes browser freezing when streaming long markdown responses by implementing incremental block-based rendering.

Problem

Markdown was re-parsed on every incoming chunk, creating slow parsing behavior. This is especially noticeable with tool calls where each result adds substantial content.

Solution

• Split content into "committed" blocks (parsed once, cached) and a "pending" tail (re-parsed on updates)
• Commit blocks at \n\n boundaries when outside code fences
• Throttle pending updates based on buffer size (larger content = less frequent parsing)
• At stream end, re-parse full content to fix any artifacts from block splitting