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.App/AboutDialog.cs

@@ -20,6 +20,31 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v1.6
+
+        Three reliability fixes. No wire-format or audio-pipeline
+        changes — v1.5 and v1.6 peers interoperate.
+
+        Bug fixes:
+          * Peer address recovery. If the address you connected
+            to goes unreachable — a peer rebooted onto a new IP,
+            or a computer name resolved to a stale address —
+            RemSound now follows the peer to the live address it
+            is heartbeating from, instead of sending audio into
+            the void. Recovers on its own within a few seconds.
+            Limited to private-network addresses so a relay can
+            never be mistaken for a moved peer.
+          * Fixed a crash that could happen when a peer
+            reconnected (e.g. after rebooting). The Connectivity
+            peer list could be read mid-rebuild with a stale
+            index and bring the app down from the status timer.
+          * Fixed runaway memory and CPU on a long-running
+            receiver. Decoder sessions orphaned by peer
+            reconnects were not being reclaimed — over hours they
+            piled up, each holding a multi-megabyte buffer and
+            costing render-thread time every callback. They are
+            now reaped once idle, with a hard cap as a backstop.
+
         RemSound v1.5
 
         Menu reorganisation, multi-peer audio-routing fix, recording