Render streamed Markdown incrementally

[copyberry]

Render streamed Markdown incrementally

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.