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/MainForm.cs

@@ -17,7 +17,8 @@ public partial class MainForm : Form
 
     public MainForm()
     {
-        Text = "gatuna";
+        var ver = System.Reflection.Assembly.GetExecutingAssembly().GetName().Version!;
+        Text = $"gatuna {ver.Major}.{ver.Minor}";
         Width = 520;
         Height = 420;
         FormBorderStyle = FormBorderStyle.FixedSingle;