fix: register-new-then-remove-old in ReplaceTrackAudioAsync; replace wording in timeout messages; doc comment on ExistingTrackCount

On partial failure the old path deleted the original audio before
confirming the new write succeeded. Now: load old extension, register
new audio first (original untouched on failure), then clean up stale
backing file only on success and only when extension changed.

DeepDrftTests/TrackReplaceAudioTests.cs

@@ -90,7 +90,8 @@ public class TrackReplaceAudioTests
         var content = CreateContentService(fileDatabase!);
 
         // A .wav original replaced by a .flac: the backing filename is keyed by extension, so a
-        // register-only swap would strand the old .wav. The replace removes the old entry first.
+        // register-only swap would strand the old .wav. The replace writes the new entry first, then
+        // cleans up the stale old backing file (detected by comparing old vs. new extension).
         var original = await WriteWavAsync(BuildMinimalPcmWav(2.0), ".wav");
         var seeded = await content.AddTrackAsync(original, "Original", "Artist");
         var entryKey = seeded!.EntryKey;