A `$$math` block whose opening line carries math and whose closing `$$` sits
at the end of the last math line parsed as an unclosed flow:
micromark-extension-math reads the opening line's text as fence meta and
accepts a closing fence only on a line whose content is exactly `$$`. The
flow then consumed the following document, and KaTeX rendered the block plus
that swallowed text as one error span. Fence meta was also silently dropped
because the renderer reads only the math node's value.
The compatibility layer's dollar flow now owns every `$$` fence, spans
multiple lines, and stops its paragraph-interruption check before the line
ending, as the upstream flow does; registering it after `math()` is what
gives it that precedence in micromark.
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