httpdl: unify completed counter and dedupe mirror loop; engine: harden untrack

From a Rob Pike pass over the tree:
- singleOnce adds deltas to d.completed like pump does, instead of
  storing absolute offsets, so the counter has one write discipline.
- one overMirrors helper replaces the three copy-pasted mirror-fallover
  loops in probeRetry, fetchSeg and single.
- failedDownload becomes a pointer, the only value-type Download impl, so
  Engine.untrack's == is always pointer identity and can't compare the
  fields of a value (keying on Stat().ID is out — a torrent's ID changes
  from source to infohash once metadata loads).

main.go

@@ -190,9 +190,9 @@ type failedDownload struct {
 	err  error
 }
 
-func (f failedDownload) Name() string              { return f.name }
-func (f failedDownload) Run(context.Context) error { return f.err }
-func (f failedDownload) Stat() download.Stat {
+func (f *failedDownload) Name() string              { return f.name }
+func (f *failedDownload) Run(context.Context) error { return f.err }
+func (f *failedDownload) Stat() download.Stat {
 	return download.Stat{Name: f.name, Status: download.Errored, Total: -1}
 }
 
@@ -281,7 +281,7 @@ func build(opts *cli.Options, targets []target) ([]job, *torrent.Client, error)
 		u := t.uris[0] // primary: classifies the target and names the session source
 		src := strings.Join(t.uris, "\t")
 		if first, isDup := dups[u]; isDup {
-			out = append(out, job{source: src, dl: failedDownload{
+			out = append(out, job{source: src, dl: &failedDownload{
 				name: u,
 				err:  fmt.Errorf("%w: same torrent as %s", errDuplicate, first),
 			}})
@@ -397,7 +397,7 @@ func followUps(jobs []job, client *torrent.Client, bo bt.Options) []job {
 		// from under the other, stranding it at 0%. build() dedupes pass-1 sources
 		// but cannot reach a not-yet-fetched HTTP .torrent, so dedupe here too.
 		if first, dup := markInfoHash(seen, path); dup {
-			out = append(out, job{source: path, dl: failedDownload{
+			out = append(out, job{source: path, dl: &failedDownload{
 				name: path,
 				err:  fmt.Errorf("%w: same torrent as %s", errDuplicate, first),
 			}})