Tests: audio-correctness + infra pinning, group 1 (review Phase 2)

Five subsystems that had ZERO coverage - all pure code where a real bug would ship
green because the flow tests only prove packets move, not that bytes are right:
- AudioRingBuffer: wrap correctness, overflow drops OLDEST, DropOldest/TrimFromProducer
  semantics, silence-fill + exact underrun/drop counters, oversized-write degrade.
- PcmPack 24-bit codec: +/-1-LSB round-trip incl. negatives (a sign-extension bug =
  distortion on the whole PCM path), sign preservation, over-range clamps not wraps.
- RemSoundUpdater tag filtering: server-v* relay tags rejected, numeric ordering
  (v5.10 > v5.9), suffix tolerance - what stops a wrong-version auto-update.
- RouterPortMapper.IsCgnatAddress: RFC 6598 boundaries exact (made internal for test).
- NetworkPriority.TryAttach: no-throw smoke on a bound socket.

Gate 52/52.

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

src/RemSound.App/RouterPortMapper.cs

@@ -321,7 +321,7 @@ internal sealed class RouterPortMapper : IDisposable
         }
     }
 
-    private static bool IsCgnatAddress(IPAddress addr)
+    internal static bool IsCgnatAddress(IPAddress addr) // internal for the self-test (RFC 6598 range pinning)
     {
         if (addr.AddressFamily != System.Net.Sockets.AddressFamily.InterNetwork) return false;
         var b = addr.GetAddressBytes();