Vital sharing across computers through the backend

Outgoing, the original's share_* frames byte for byte: subscribe with the
player id on start and without it after every reconnect, vitals on change or
every 3 s, position on change or every 10 s, the item heartbeat every 5 s,
unsubscribe on stop. Debuff casts come from the host's peer network: the
attempts and landings the bot announces, plus the player's own completed
casts, so a hand-cast debuff is shared once it lands (a plugin cannot see
outgoing packets, so hand-cast attempts are not). Ids stay signed ints, skill
a double, duration_ms the spell's duration.

Incoming, another computer's characters and casts go into the host's peer
network (the new relay imports), where the bot reads them exactly as it reads
characters on this computer: vitals and position for heals, landed debuffs
for target choice. Peers not heard from for 15 s drop out; the original kept
them forever. Logoff now runs features in reverse so the unsubscribe leaves
before the socket closes, and stopping the socket flushes what was queued.

Builds against a preview of the plugin API with the peer relay
(0.1.19-mmpreview.1, packed from the relay commit); switch to the released
package when it exists.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/OpenAC.MosswartMassacre/FeatureCatalog.cs

@@ -29,5 +29,11 @@ internal static class FeatureCatalog
         features.Add(new BotControl(context));
         features.Add(new Views.MainWindow(context, stats));
         session.AddConnectionListener(telemetry);
+
+        // Cross-computer vital and debuff sharing.
+        var sharing = new Sharing.VitalSharing(context);
+        features.Add(sharing);
+        session.AddConnectionListener(sharing);
+        session.AddShareHandler(sharing);
     }
 }