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

README.md

@@ -181,6 +181,24 @@ gatuna/
     └── PingTest.cs          # PING/PONG network test with stats
 ```
 
+## Versioning
+
+Both programs use `MAJOR.MINOR.PATCH` where:
+
+- **MAJOR** matches the wire protocol version. A v2.x program rejects any frame
+  with `version != 2`. No cross-major compatibility.
+- **MINOR** is the main change indicator for features and fixes within a major
+  version.
+- **PATCH** is reserved for bug fixes; generally unused.
+
+Current version: **2.0.0** (wire protocol v2).
+
+Check versions:
+```sh
+gatunad --version          # Rust server
+```
+The Windows client shows its version in the title bar and tray tooltip.
+
 ## License
 
 CC0 1.0 Universal. See [`LICENSE`](LICENSE).