Add streamdown — streaming markdown renderer for Flutter AI apps

[jayu1023]

Hi! Adding a new entry under Markdown:

streamdown — a flicker-free streaming markdown renderer purpose-built for Flutter AI chat apps. It replaces flutter_markdown's "re-parse every chunk" approach with an append-only AST + stable widget keys, so closed blocks never re-render mid-stream.

• 📦 pub.dev: https://pub.dev/packages/streamdown (160/160 pub points)
• 🐙 Source: https://github.com/jayu1023/streamdown
• 📊 Benchmark: ~188× faster than naive re-parse on 5KB markdown streamed in 4-char chunks
• ⚙️ Drop-in API: Streamdown(stream: openai.responseStream)
• 🪪 License: BSD-3-Clause

Demo GIF: https://github.com/jayu1023/streamdown/blob/main/example/screenshots/split_screen_demo.gif

Thanks for maintaining this list!

[jayu1023]

Closing as a duplicate — superseded by #1036 which is the active streamdown submission. Sorry for the noise.