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.

tests/GameCult.Caching.Tests/README.md

@@ -0,0 +1,15 @@
+# GameCult.Caching.Tests
+
+`GameCult.Caching.Tests` contains NUnit tests for the caching library and its persistence abstractions.
+
+## Scope
+
+The tests currently cover:
+
+- rename behavior in `MultiFileNewtonsoftJsonBackingStore`
+
+## Run
+
+```powershell
+dotnet test tests\GameCult.Caching.Tests\GameCult.Caching.Tests.csproj
+```