library: finish the credit query review nits

Put the download-visibility comment on the constant it describes, keep the
no-scan guards beside the seek assertions, and give the identical-sync test a
positive control for both the artist aggregate and the album play recompute.

app/src/test/kotlin/com/subtracks/data/db/LibraryDaoTest.kt

@@ -871,6 +871,8 @@ class LibraryDaoTest {
                 plan,
                 plan.contains("USING INDEX index_song_artists_sourceId_artistId (sourceId=? AND artistId=?)"),
             )
+            assertFalse(plan, plan.contains("SCAN albums"))
+            assertFalse(plan, plan.contains("SCAN songs"))
         }
 
     private suspend fun plan(sql: String): String =

app/src/test/kotlin/com/subtracks/data/sync/SyncServiceTest.kt

@@ -1765,7 +1765,7 @@ class SyncServiceTest {
         }
 
     @Test
-    fun anIdenticalSyncSkipsTheArtistAggregateQuery() =
+    fun anIdenticalSyncSkipsTheAggregateQueries() =
         runTest {
             val context = ApplicationProvider.getApplicationContext<Context>()
             val executions = Collections.synchronizedList(mutableListOf<SqlExecution>())
@@ -1786,11 +1786,16 @@ class SyncServiceTest {
                     )
 
                 SyncService(loggingDb, source).sync()
+                val firstSyncExecutions = synchronized(executions) { executions.toList() }
+                assertTrue(firstSyncExecutions.any { it.sql.contains("credited_albums") })
+                assertTrue(firstSyncExecutions.any { it.sql.contains("UPDATE albums SET playCount") })
+
                 synchronized(executions) { executions.clear() }
                 SyncService(loggingDb, source).sync()
 
                 val secondSyncExecutions = synchronized(executions) { executions.toList() }
                 assertTrue(secondSyncExecutions.none { it.sql.contains("credited_albums") })
+                assertTrue(secondSyncExecutions.none { it.sql.contains("UPDATE albums SET playCount") })
             } finally {
                 loggingDb.close()
             }