address review: mark the fetch bound test-only; fix the concurrency test

- The concurrency bound already existed; this parameter only exists so tests
  can force the sequential path, and no production path passes it. Mark it
  `@VisibleForTesting` so that is explicit rather than reading as a wired
  setting.
- `playlistSongsUseTheSameRowsAtAnyFetchBound` could not fail (bound has no
  effect on the rows) — delete it and its `normalized` helper.
- `playlistSongsRespectsASmallerFetchBound` relied on a 300ms overlap
  window; use latches so the bound is proven deterministically (fails when
  the semaphore ignores the bound).

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicSource.kt

@@ -1,5 +1,6 @@
 package com.subtracks.data.source.subsonic
 
+import androidx.annotation.VisibleForTesting
 import com.subtracks.data.model.Album
 import com.subtracks.data.model.Artist
 import com.subtracks.data.model.Playlist
@@ -34,6 +35,7 @@ class SubsonicSource(
     private val maxBitrate: Int = 0,
     private val streamFormat: String? = null,
     private val maxPages: Int = MAX_PAGES,
+    @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
     private val maxConcurrentFetches: Int = MAX_CONCURRENT_FETCHES,
 ) : MusicSource {
     private var emptyQuerySearchSupported: Boolean? = null