Bump to v1.6.0: peer-address recovery, reconnect crash, long-run memory/CPU leak

Three reliability fixes. Wire format and audio pipeline unchanged from
v1.4 / v1.5 — all interoperate.

Peer address recovery:
* When a tracked peer goes Unreachable (its resolved address — often a
  stale DNS / Pi-hole record, or a peer that rebooted onto a new IP) but
  the same peer is still heartbeat-pinging us from a different address,
  RemSound now adopts the live address instead of transmitting to a dead
  one. HeartbeatService records untracked ping sources; MainForm's
  TryAdoptLiveHeartbeatAddress (1 Hz) re-points the sender, heartbeat
  tracking and receiver allow-list. Conservative: fires only on the
  unambiguous one-unreachable-and-one-source case, private-range (RFC1918)
  addresses only so a relay can't hijack the sender, 10 s cooldown.

Reconnect crash:
* Fixed IndexOutOfRangeException in MainForm.SyncConnectedList. A churny
  peer-list rebuild (peer reboot) left SelectedIndex pointing past the
  rebuilt item array; the 1 Hz status timer read SelectedItem and crashed
  the app. New SafeSelectedItem bounds-checks the index; applied to all
  three timer-driven sync methods. The status tick is also wrapped in
  try/catch so a transient WinForms hiccup logs instead of crashing.

Long-run memory / CPU leak:
* A receiver left running for hours grew to gigabytes and climbing CPU.
  Decoder sessions orphaned by peer reconnects were not reaped — every
  reconnect mints a fresh (endpoint, streamId) key, and PruneIdleSessions
  silently skipped sessions whose PlayoutEngine lookup missed. Rewrote it
  to reap on each session's own LastWriteUtc (no cross-dictionary lookup),
  added a hard MaxLiveSessions cap as a backstop, and a "stream sessions
  live: N" diagnostic line. Bounds both memory and render-thread CPU.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/RemSound.Receiver/StreamSession.cs

@@ -34,6 +34,13 @@ internal sealed class StreamSession : IDisposable
     public AudioFormatInfo Format { get; }
     public AudioTransportCodec Codec => (AudioTransportCodec)Format.Codec;
 
+    /// <summary>UTC timestamp of the most recent decoded-audio write into this session's
+    /// playout buffer. <see cref="AudioReceiver.PruneIdleSessions"/> reaps on this directly,
+    /// rather than a cross-dictionary lookup into PlayoutEngine that could miss and strand
+    /// the session forever — a reconnecting peer never reuses its old (endpoint, streamId)
+    /// key, so its previous session is always an orphan that must be reaped by idle age.</summary>
+    public DateTime LastWriteUtc => sessionPlayout.LastWriteUtc;
+
     /// <summary>For PCM streams: number of incoming packets the assembler rejected outright.</summary>
     public long PcmFrameRejections => pcmAssembler.RejectionCount;
     /// <summary>For PCM streams: number of partially-assembled frames discarded mid-flight.</summary>