version both programs at 2.0.0 (wire protocol v2)

Versioning scheme: MAJOR.MINOR.PATCH where MAJOR matches the wire
protocol version (no cross-major compat), MINOR is the main change
indicator, PATCH generally unused.

- gatunad: Cargo.toml version 2.0.0 (clap --version picks it up)
- gatuna: csproj Version 2.0.0, shown in form title + tray tooltip
- app.manifest: assemblyIdentity version 2.0.0.0
- Frame version rejection already in place (both sides reject != 2)
- README: document versioning scheme

gatuna-win/Program.cs

@@ -9,10 +9,12 @@ static class Program
 
         var form = new MainForm();
 
+        var ver = System.Reflection.Assembly.GetExecutingAssembly().GetName().Version!;
+
         using var tray = new NotifyIcon
         {
             Icon = SystemIcons.GetStockIcon(StockIconId.NetworkConnect, 32),
-            Text = "gatuna",
+            Text = $"gatuna {ver.Major}.{ver.Minor}",
             Visible = true,
         };