[Linuxuser67]

Second transport migration (follows #44's part downloads; the two touch disjoint hunks and merge in either order):

• finish_merge runs the combine through ffmpeg directly: same -i audio/-i video, -map 0:a/1:v, -c:v copy, copy-or-AAC audio matrix ported from the crate's audio_codec_for_mux, title embedded in the same pass (replacing its warn-only second-pass tagger). Failures now surface ffmpeg's own last line — the YouTube AV1 report will name its cause instead of a wrapped crate error.
• fetch_video_page spawns yt-dlp with tokio + timeout instead of the crate Executor: same semantics (concurrent pipe drain for multi-MB JSON, timeout kill, nonzero exit as error) with stderr detail on failure and stdin nulled.

Tests: codec matrix, argv mapping/title/separator, fake-ffmpeg merge success + failure detail, fake-yt-dlp extraction parse. Full suite green, clippy clean. Downloader construction stays for now (parts still use its manager until #44 lands); selectors, models and the installer are untouched by design.