Phase 0: scaffold modern RIOJoy solution + plan

Modernization of the legacy vJoy-based RIO cockpit interface for Win10/11,
removing the vJoy dependency in favor of a custom VHF/UMDF HID driver,
rewritten in C#/.NET 8 as a background tray app with per-game profiles.

- Reorganize: legacy C++ -> legacy/, cockpit art -> docs/reference/
- RioJoy.sln: src/RioJoy.Core (lib) + src/RioJoy.Tray (tray app), net8.0-windows x64
- driver/ placeholder for the RioGamepad WDK driver
- docs/PLAN.md (7-phase plan; profiles + serial-yield model)
- docs/PROTOCOL.md (RIO wire format + iRIO input-map reference)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

RioJoy.sln

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