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.
Loretta
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