downloader: comprehensive snapshot set, per-snapshot progress output

Snapshot discovery now takes the union of:
- All names in preverified.toml
- All files on disk with ".torrent" stripped
- All files on disk with ".part" stripped

This lets the verifier detect missing .torrent files, infohash mismatches
between preverified.toml and .torrent, .part-vs-full disagreements, and
data files that are absent entirely.

After each snapshot finishes verification, a {"type":"progress",...} line
is written to stdout with the snapshot name, overall % complete, and the
preverified status ("matches" | "doesn't match" | "not in preverified" |
"no preverified.toml"). Issue and summary lines retain their own types.

Author: anacrolix

cmd/utils/app/snapshots_cmd.go

@@ -557,7 +557,7 @@ var snapshotCommand = cli.Command{
 					return fmt.Errorf("verify snapshot downloads: %w", err)
 				}
 				log.Root().Info("[snapshots verify] done",
-					"torrents", summary.Torrents,
+					"snapshots", summary.Total,
 					"errors", summary.Errors,
 					"warnings", summary.Warnings,
 				)