refactor: delete dead code, dedupe helpers, add tests for untested packages

Dead code
  Removed, each verified to have no non-test caller: promptSearchSelection
  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,
  download.NewWithVerifySSL, qobuz.Client.GetTrackMetadata,
  tidal.Client.trackSupportsAtmos (test-only, and the comment above
  GetDownloadable already documented its removal from the live path) and its
  now-orphaned test, yandex.canonicalTrackRequestID (a no-op returning its
  trimmed input; the call site uses strings.TrimSpace directly) and the unused
  defaultEstimatedKbps constant.

Config keys that did nothing
  qobuz.download_booklets and misc.check_for_updates were declared, defaulted
  and documented in config.toml.example but read by no code. The whole
  [youtube] block went too: there is no YouTube provider. go-toml ignores
  unknown keys, verified, so existing user configs still load.

Duplicated helpers
  tidal.stringify and yandex.firstNonEmpty were byte-identical copies of
  jsonutil.StringFromAny and jsonutil.FirstNonEmpty. displayArtistNames
  existed three times, byte-identical, in internal/app, provider/yandex and
  provider/beatport; it now lives in jsonutil. The CLI's normalizeCodec
  delegates to config.NormalizeCodec so the flag and the config file are
  checked by the same rules.

Tests for packages that had none
  internal/jsonutil (0% -> 100%) and internal/netutil (0% -> 78%), the latter
  covering redactURL, which is a security control, and the zero-timeout
  contract that internal/download and the qobuz body copy depend on.
  internal/config rose to 82% with coverage of the new Validate(), the
  0600 permissions, CloneConfigData not sharing slices, and a concurrent
  UpdateTokens test that exercises the mutex under -race.

Test suite is ~85x faster
  Provider tests built clients with the default 60 requests/minute, so the
  rate limiter slept 1s per HTTP call: deezer took 31s, tidal 11s, qobuz 8s.
  Each provider test package now builds its config through a testConfigData
  helper that disables throttling. Whole suite: ~60s -> 0.7s, and 2.8s
  under -race.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

cmd/rip/search.go

@@ -110,84 +110,6 @@ func parseSearchArgs(args []string, defaultLimit int) (searchOptions, error) {
 	}, nil
 }
 
-func promptSearchSelection(results []searchResult) ([]int, error) {
-	reader := bufio.NewReader(os.Stdin)
-	for {
-		fmt.Print("Select results to download (e.g. 1,3-5; a=all; q=cancel): ")
-		line, err := reader.ReadString('\n')
-		if err != nil {
-			return nil, err
-		}
-		line = strings.TrimSpace(line)
-		if line == "" || strings.EqualFold(line, "q") || strings.EqualFold(line, "quit") {
-			return nil, nil
-		}
-		if strings.EqualFold(line, "a") || strings.EqualFold(line, "all") {
-			out := make([]int, 0, len(results))
-			for i := range results {
-				out = append(out, i)
-			}
-			return out, nil
-		}
-
-		selected := map[int]struct{}{}
-		chunks := strings.Split(line, ",")
-		ok := true
-		for _, raw := range chunks {
-			part := strings.TrimSpace(raw)
-			if part == "" {
-				continue
-			}
-			if strings.Contains(part, "-") {
-				bounds := strings.SplitN(part, "-", 2)
-				if len(bounds) != 2 {
-					ok = false
-					break
-				}
-				start, err1 := strconv.Atoi(strings.TrimSpace(bounds[0]))
-				end, err2 := strconv.Atoi(strings.TrimSpace(bounds[1]))
-				if err1 != nil || err2 != nil || start <= 0 || end <= 0 || start > end {
-					ok = false
-					break
-				}
-				for i := start; i <= end; i++ {
-					if i > len(results) {
-						ok = false
-						break
-					}
-					selected[i-1] = struct{}{}
-				}
-				if !ok {
-					break
-				}
-				continue
-			}
-			idx, err := strconv.Atoi(part)
-			if err != nil || idx <= 0 || idx > len(results) {
-				ok = false
-				break
-			}
-			selected[idx-1] = struct{}{}
-		}
-
-		if !ok || len(selected) == 0 {
-			fmt.Println("Invalid selection, try again.")
-			continue
-		}
-
-		out := make([]int, 0, len(selected))
-		for idx := range selected {
-			out = append(out, idx)
-		}
-		for i := 1; i < len(out); i++ {
-			for j := i; j > 0 && out[j] < out[j-1]; j-- {
-				out[j], out[j-1] = out[j-1], out[j]
-			}
-		}
-		return out, nil
-	}
-}
-
 func promptSearchSelectionMenu(source, mediaType, query string, results []searchResult) ([]int, error) {
 	if len(results) == 0 {
 		return nil, nil