probeVersion() now captures stderr/exit-code/signal/spawn-error instead of
returning a bare boolean, and ensureYtDlp() tries the bundled binary first,
falls back to `yt-dlp(.exe)` on PATH if the bundled one won't execute (AV
block, missing libc symbol, broken download), and only then re-downloads.
The final user-facing error includes the per-attempt diagnostics so we can
actually see WHY verification failed instead of the opaque
"yt-dlp 다운로드는 됐지만 실행 검증에 실패했습니다." message.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>