CVE-2026-50158: yutu: Arbitrary File Write via MCP `caption-download` Tool

The caption-download MCP tool in yutu passes the caller-supplied file parameter directly to os.Create() at pkg/caption/caption.go:272 without any path validation, canonicalization, or confinement to the pkg.Root boundary (YUTU_ROOT). A local attacker — or any process able to reach the HTTP MCP server — can write arbitrary content to any path writable by the yutu process, entirely outside the intended working directory. This is a High severity vulnerability (CVSS 7.7) with high integrity and availability impact.