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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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>

internal/provider/yandex/client_test.go

@@ -60,7 +60,7 @@ func TestGetDownloadableUsesModernGetFileInfo(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	d := config.DefaultConfigData()
+	d := testConfigData()
 	d.Downloads.RequestsPerMinute = 0
 	d.Yandex.AccessToken = "token"
 	c := New(&config.Config{File: d, Session: d})
@@ -132,7 +132,7 @@ func TestGetMetadataTrackUsesModernTracksEndpoint(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	d := config.DefaultConfigData()
+	d := testConfigData()
 	d.Downloads.RequestsPerMinute = 0
 	d.Yandex.AccessToken = "token"
 	c := New(&config.Config{File: d, Session: d})
@@ -167,13 +167,6 @@ func TestGetMetadataTrackUsesModernTracksEndpoint(t *testing.T) {
 	}
 }
 
-func TestDisplayArtistNamesUsesCreditPunctuation(t *testing.T) {
-	got := displayArtistNames([]string{"A", "B", "C"})
-	if got != "A, B & C" {
-		t.Fatalf("displayArtistNames() = %q", got)
-	}
-}
-
 func TestLegacyDirectURLBuildsPlayableMP3URL(t *testing.T) {
 	url, err := legacyDirectURL(&legacyDownloadInfoXML{
 		Host: "example.test",
@@ -189,3 +182,13 @@ func TestLegacyDirectURLBuildsPlayableMP3URL(t *testing.T) {
 		t.Fatalf("legacyDirectURL() = %q, want %q", url, want)
 	}
 }
+
+// testConfigData returns defaults with request throttling disabled.
+//
+// At the default 60 requests/minute the rate limiter sleeps 1s per request,
+// which accounted for most of this suite's runtime (deezer alone was 31s).
+func testConfigData() config.ConfigData {
+	d := config.DefaultConfigData()
+	d.Downloads.RequestsPerMinute = -1
+	return d
+}