Skeleton: plugin root, backend socket, /mm router, settings, test harness

The socket connects with the shared-secret header, registers first, reassembles
incoming frames of any size (the original dropped anything over 4 KiB) and
reconnects two seconds after a failure. Incoming share_* frames go to the
vital-sharing handlers; any other frame is a chat-box command for this
character, run one per tick through the chat bar as if typed. Frames are
written with Newtonsoft.Json defaults, the original's serializer, so the
backend sees the same bytes. The secret lives in the character's settings.

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

src/OpenAC.MosswartMassacre/FeatureCatalog.cs

@@ -0,0 +1,17 @@
+using OpenAC.MosswartMassacre.Backend;
+
+namespace OpenAC.MosswartMassacre;
+
+/// <summary>
+/// Every feature the plugin runs, in the order the root logs them in. Each
+/// slice of the port adds its features here and nowhere else.
+/// </summary>
+internal static class FeatureCatalog
+{
+    public static void AddFeatures(MmContext context, BackendSession session, List<IMmFeature> features)
+    {
+        _ = context;
+        _ = session;
+        _ = features;
+    }
+}