JSON backing store, I/O fixes, Networking Security

Introduced Newtonsoft.Json-backed persistence.
Improved backing-store reliability and serialization across the caching stack.
Improved networking security posture.

Key changes:
- Add new GameCult.Caching.NewtonsoftJson project with SingleFile/MultiFile backing stores and polymorphic known-type envelopes.
- Add MessagePack helper serialization (DatabaseEntrySerialization) and update MessagePack backing stores to use the shared options/resolver; add DatabaseLink formatter and generator/analyzer docs.
- Update CultLib.sln to include the new projects and new caching tests project.
- Improve CultCache persistence behavior: track pending store operations, wait for operations on PullAll, and centralize async push handling with error logging.
- Harden file I/O in multi-file/single-file stores: atomic temp-file writes, safe Replace/Move semantics, rename-safe deletes, and improved file-watcher subscription and debounce disposal.
- Add README files and documentation for several subprojects and packaging projects.

These changes add JSON persistence support, improve serialization consistency, and make on-disk operations more robust and observable.

src/GameCult.Networking/MessageSerialization.cs

@@ -0,0 +1,20 @@
+using MessagePack;
+
+namespace GameCult.Networking
+{
+    internal static class MessageSerialization
+    {
+        public static readonly MessagePackSerializerOptions Options =
+            MessagePackSerializer.DefaultOptions.WithSecurity(MessagePackSecurity.UntrustedData);
+
+        public static byte[] Serialize<T>(T message) where T : Message
+        {
+            return MessagePackSerializer.Serialize(message, Options);
+        }
+
+        public static T Deserialize<T>(byte[] payload)
+        {
+            return MessagePackSerializer.Deserialize<T>(payload, Options);
+        }
+    }
+}