fix: sweep fragment partials immediately on cancel

[Mr-Neutr0n]

Problem

Cancelling a download leaked yt-dlp's partial files in the output directory. Two stacked causes:

1. The artifact list was incomplete. Cleanup removed dest, dest.part, and dest.ytdl — but fragmented downloads write dest.part-Frag0, dest.part-Frag1, … siblings that a plain rm of the .part never touches.
2. Cleanup waited for the child's close event. spawn(bin, args, {signal}) signals only the direct child; anything it spawned (an ffmpeg merge step, a shell's sleep) inherits the stdio pipes and keeps them open after the parent dies, delaying close by the grandchild's entire runtime. Until it fires, no cleanup happens at all.

Reproduced with a scripted fake yt-dlp: cancel mid-download → .part, .part-Frag0, .part-Frag1 all still on disk a minute later.

Change

• On abort, sweep partials immediately from what the live stdout parser already collected — destinations are parsed as lines stream, so by cancel time they are known. The close handler re-sweeps for any line that streamed past the abort.
• The sweeper now enumerates the destination's sibling family: exact name, dot-suffixed (.ytdl, .part*) and dash-suffixed (-Frag<n>), while explicitly not touching similarly-named user files like <dest>-notes.txt.
• Successful downloads and genuine failures behave exactly as before.

Verification

New src/lib/ytdlp.test.ts drives the real download() against scripted fake binaries writing actual fragment files:

• cancel sweeps .part + -FragN siblings (fails on old code, passes now)
• late-announced destinations still get swept
• full suite: 8 passed (6 existing + 2 new); tsup build clean

---

This change was prepared with AI assistance under human direction and review.

[Mr-Neutr0n]

Hi @pablostanley - a gentle nudge on sweeping fragment partials on cancel; still mergeable and applies cleanly. Happy to adjust anything, and fine to close if not wanted. Thank you.