feat(audit): Batch 15 live-verify — download-engine lifecycle (R4, L65, L139, B2, B6, L143)

The watched live session for the six deferred lifecycle items, each verified
against real yt-dlp/Electron behavior on Windows:

- R4: cancel now deletes the download's orphaned partials. The engine captures
  the final output path via a new `--print before_dl:dest|%(filename)s`
  (empirically %(filepath)s is still 'NA' that early) and the close handler
  sweeps only unambiguous intermediates (.part / .part-Frag* / .ytdl /
  <stem>.fNNN.<ext>) via the pure, unit-tested lib/partials.ts — never a bare
  <stem>.<ext> or another download's files. Live-verified with decoys.
- L65: verified-acceptable, no code change — a taskkill /F pause leaves the
  .part byte-intact and yt-dlp --continue resumes at the exact byte offset.
- L139: spawn↔self-update interlock (new ytdlpLock.ts). The updater refuses
  while any yt-dlp spawn is live; download/terminal IPC handlers hold (await)
  behind an in-progress exe rewrite instead of failing.
- B2: source indexing is cancellable — AbortSignal through the probe walk
  (kills the in-flight child, live-verified via tasklist), sources:index-cancel
  IPC, and a Cancel button in the Library add-source row.
- B6: app-update download is cancellable — app:update-cancel routes through
  the existing finish() teardown (abort + destroy + unlink, live-verified on
  Electron net.request); Cancel button under the update progress bar. The
  checksum phase is cancelable too.
- L143: closing the window mid-download (non-tray mode) now offers a native
  tray-independent "Quit anyway / Keep downloading" dialog, replacing the
  passive "use the tray to quit" notification.

Peer-review pass caught and fixed: a non-reentrant update lock (concurrent
begin clobbered the settle promise), a dead Cancel window during the B6
checksum fetch + a canceller slot-ownership bug (L140 shape), and a
persist-after-cancel hole in the index walk.

typecheck + 304 tests + eslint + production build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/shared/ipc.ts

@@ -10,6 +10,8 @@ export const IpcChannels = {
   appUpdateCheck: 'app:update-check',
   /** download the latest release's installer to a temp file */
   appUpdateDownload: 'app:update-download',
+  /** abort the in-flight installer download (B6) */
+  appUpdateCancel: 'app:update-cancel',
   /** launch a downloaded installer and quit so it can replace the app */
   appUpdateRun: 'app:update-run',
   /** main → renderer push channel for installer download progress */
@@ -68,6 +70,8 @@ export const IpcChannels = {
   sourcesList: 'sources:list',
   sourceIndex: 'sources:index',
   sourceReindex: 'sources:reindex',
+  /** renderer → main: abort the in-flight index/reindex walk (B2) */
+  sourceIndexCancel: 'sources:index-cancel',
   sourceRemove: 'sources:remove',
   sourceItems: 'sources:items',
   /** mark a media item downloaded once its collection download completes */