The anchored fenced-code regex ended the closing fence with [ \t]*$, so a
close line terminated by CRLF (\r\n) — Windows-authored content — failed to
match: the \r sits between the fence and the $ position. That left the whole
block unprotected, escaping its backticks into literal ` runs and dropping
the code formatting. Allow an optional trailing \r before the line end.
Adds a CRLF-fence regression (fails on the old anchor).
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