GHSA-hjh7-r5w8-5872: SiYuan: Path Traversal via Double URL Encoding in `/export/` Endpoint (Incomplete Fix Bypass for CVE-2026-30869)

The fix for CVE-2026-30869 in SiYuan v3.5.10 only added a denylist check (IsSensitivePath) but did not address the root cause — a redundant url.PathUnescape() call in serveExport(). An authenticated attacker can use double URL encoding (%252e%252e) to traverse directories and read arbitrary workspace files including the full SQLite database (siyuan.db), kernel log, and all user documents.