CVE-2026-71212: xidown: Argument Injection via Unterminated yt-dlp Command Line Construction

xidown (a yt-dlp/ffmpeg GUI wrapper) builds its yt-dlp command-line invocation (xidown/core/scanner.py and downloader.py) by appending the user-provided or scanned URL as a bare trailing positional argument, with no '--' end-of-options marker and no scheme validation anywhere in the codebase. Because yt-dlp parses any argument beginning with '-' as a CLI option rather than link text, a crafted 'URL' value such as -U (yt-dlp's self-update flag) or --exec=... is parsed as a real yt-dlp option instead of a URL, altering the tool's control flow before its own URL validation runs. Full code execution via --exec was not demonstrated in the single-URL flow tested, but the underlying argument-injection primitive is confirmed and unmitigated across all call sites.