fix: simplify regex to avoid ReDoS vulnerability

Author: nikolay-e

src/treemapper/diffctx/fragments.py

@@ -694,7 +694,7 @@ def _find_section_end(self, lines: list[str], start_line: int, level: int, remai
 
 class RegexMarkdownStrategy:
     priority = 85
-    _HEADING_RE = re.compile(r"^(#{1,6})\s+([^\n]+)$", re.MULTILINE)
+    _HEADING_RE = re.compile(r"^(#{1,6}) (.+)$")
 
     def can_handle(self, path: Path, content: str) -> bool:
         return path.suffix.lower() in {".md", ".markdown", ".mdx"}