The code fence (```) was not being properly detected during streaming,
causing it to be rendered as inline code instead of a code block.
Root cause: When buffering a code fence after seeing ```, the code
was returning early for ALL characters including newlines. This meant
handle_newline() was never called and block_state was never set to
BlockState::CodeBlock.
Fixes:
- Don't return early for newlines when buffering code fence, allow them
to fall through to handle_newline()
- Support indented code fences (up to 3 spaces per CommonMark spec) by
using trim_start() when checking for ``` at line start
fc9a2f835a