When streaming markdown headers containing inline tags (backticks, bold,
italic), the closing delimiter triggered early emission via
emit_formatted_inline(). Since format_header() appends a newline, any
text after the closing tag ended up on a separate line.
Added an in_header guard to handle_delimiter() so headers wait for the
actual newline to emit as a complete line. Added 4 char-by-char streaming
tests covering the bug pattern.
e30ddb8cbc