fix(tui_v2): render markdown while streaming

[MuziIsabel]

Summary

• Render assistant Markdown during streaming with stable-prefix caching and bounded tail parsing.
• Temporarily close open fenced code blocks so live code output is styled before completion.
• Keep completed responses on the existing full Markdown render path.

Test plan

• python -m py_compile e:\Work\Python\GenericAgent-live-md-pr\frontends\tuiapp_v2.py
• Helper smoke test for heading split, open fence provisional closure, and table tail split.