Denial of service via quadratic Markdown parsing

Algorithmic complexity in GitHub cmark-gfm 0.29.0.gfm.9 and earlier allows attackers to cause a service hang with crafted Markdown. The block parser's check_open_blocks loop rescans every open ancestor for each empty line after a deeply nested list, while blank-line state propagation repeatedly revisits ancestors in nested block quotes. Exploitation requires an application to pass attacker-controlled Markdown to the parser; processing time grows quadratically until service work is delayed or halted.