Refactor docs: topic folders, TOON, XCUT, protocol sync
- Migrated all topic documentation into dedicated folders with canonical `README.md`, `ISSUES.md`, and `TODO.md` per topic (e.g., `LOGGING/`, `SIGNALR/`, `BINARY/`, `TOON/`).
- Added comprehensive TOON serializer documentation: design, format, options, attributes, inference, issues, and TODOs.
- Introduced `XCUT` folder for cross-cutting issues and TODOs, with canonical entries and topic cross-references.
- Updated all references and navigation to use new folder-based doc paths; fixed links and clarified doc structure.
- Enhanced AI agent protocol: enforce session skill preloading, `[LOADED_DOCS: ...]` short-name prefix, and mandatory `docs-check` skill for doc/code sync.
- Updated `.csproj` to include all `README.md` files for IDE visibility.
- Improved and clarified SignalR, grid, and project-level documentation.
- Minor code/test tweaks and doc content corrections for consistency.
- Updated all projects to use `Newtonsoft.Json` v13.0.3 for consistency.
- Introduced `HybridReferenceResolver` for semantic and numeric ID handling.
- Refactored `SerializeObjectExtensions` to support deep JSON merging.
- Simplified `IId<T>` interface by removing `IEquatable<T>` constraint.
- Improved `AcSignalRDataSource` with robust `AddRange` and `CopyTo` methods.
- Added `JsonExtensionTests` for deep hierarchy, reference, and edge cases.
- Implemented `UnifiedMergeContractResolver` for custom JSON behavior.
- Optimized type/property caching with `TypeCache` and `CachedPropertyInfo`.
- Enhanced SignalR integration to fix primitive array deserialization issues.
- Introduced `JsonNoMergeCollection` attribute for replace-only collections.
- Added test DTOs and `TestDataFactory` for real-world scenario simulations.
- Improved performance with `ConcurrentDictionary` and `ObjectPool`.
- Fixed `$id`/`$ref` handling for non-semantic references and arrays.