Application slowdown via newline-rule trailing-space regex

Inefficient regular expression processing in markdown-it before 12.3.2 allows remote attackers to slow applications via crafted Markdown. The inline newline rule applies a trailing-space regular expression to an accumulated pending string; long runs of spaces force excessive repeated scanning before a hard break. Exposure requires a consuming application to parse attacker-controlled Markdown containing the pathological pattern, and the bounded consequence is excessive CPU use and service slowdown rather than data access or modification.