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>

src/RioJoy.Tray/TrayApplicationContext.cs

@@ -0,0 +1,42 @@
+namespace RioJoy.Tray;
+
+/// <summary>
+/// Owns the tray icon and (eventually) the RIOJoy runtime: the serial link to
+/// the RIO, the active profile, the virtual-HID feeder, and the profile
+/// auto-switch watcher.
+///
+/// Phase 0 scaffold: just the tray icon + a minimal menu. Wiring the runtime is
+/// Phase 5 work; the menu items below are placeholders that mirror the legacy
+/// console menu (reset/recalibrate axes, version/status, quit).
+/// </summary>
+internal sealed class TrayApplicationContext : ApplicationContext
+{
+    private readonly NotifyIcon _trayIcon;
+
+    public TrayApplicationContext()
+    {
+        var menu = new ContextMenuStrip();
+        menu.Items.Add("RIOJoy (scaffold)").Enabled = false;
+        menu.Items.Add(new ToolStripSeparator());
+        menu.Items.Add("Exit", null, (_, _) => ExitThread());
+
+        _trayIcon = new NotifyIcon
+        {
+            Icon = SystemIcons.Application,
+            Text = "RIOJoy",
+            Visible = true,
+            ContextMenuStrip = menu,
+        };
+    }
+
+    protected override void Dispose(bool disposing)
+    {
+        if (disposing)
+        {
+            _trayIcon.Visible = false;
+            _trayIcon.Dispose();
+        }
+
+        base.Dispose(disposing);
+    }
+}