Retire the Qt/C++ host; ship the Avalonia host

Delete the Qt host (src/, cmake/, CMakeLists.txt, find-qt.bat, check-setup.bat,
build.bat, deploy.bat) now that the Avalonia host in host/ covers it on the same
dotnet toolchain. Add a self-hosted host-build CI job and a host README, and
rewrite the top-level README for the C# server + Avalonia host layout.

README.md

@@ -1,55 +1,42 @@
-# Commonwealth Online Server and Qt Host
+# Commonwealth Online — Server & Host
 
-This repository contains the Commonwealth Online authoritative dedicated server and the Qt Host GUI.
+The dedicated server and its host GUI for Commonwealth Online (Fallout 4
+multiplayer). Everything now builds on one .NET 8 toolchain.
 
-The dedicated server is C#/.NET. Valve GameNetworkingSockets remains behind the small native C++ bridge in `server/native_transport/`.
+## Components
 
-## Server
+- **[`server/`](server/README.md)** — the dedicated relay server
+  (`CommonwealthOnline.Server`, a .NET 8 console app). Run it with
+  `server/start.sh` / `server/start.bat`, or `CommonwealthOnline.Server serve`.
+- **[`host/`](host/README.md)** — the **Avalonia** cross-platform host GUI:
+  start/stop the server, edit config, watch the log, and manage players
+  (kick/ban). Replaces the former Qt/C++ host.
+
+## Quick start — server
 
 ```bash
 cd server
-dotnet build CommonwealthOnline.Server.csproj -c Release
-dotnet run --project tests/CommonwealthOnline.Server.Tests.csproj -c Release
-dotnet run --project CommonwealthOnline.Server.csproj -- serve --config commonwealth-server.json --interactive
+./start.sh          # Linux / macOS
+start.bat           # Windows
 ```
 
-The server owns Protocol V2 admission, server-owned player IDs, packet validation, movement validation, interest filtering, durable player state, scoped NPC authority and epochs, combat routing, world state, bans, rate limits, LAN discovery and localhost administration.
+Needs the .NET 8 runtime (or the SDK for a source checkout). Allow TCP `7777`
+(and optionally UDP `7778` for LAN discovery) through the firewall. `0.0.0.0`
+is a bind address, not the address players join.
 
-Transport policy remains:
+## Quick start — host GUI
 
-- `transform`, `npcState`: unreliable/sequenced under GNS
-- session/control, player state, combat, world state and authority: reliable/ordered
-
-TCP compatibility keeps newline framing inside the TCP transport only. GNS is message-oriented and uses the `COG2` snapshot sequence envelope for latest-wins snapshots.
-
-## Native GNS bridge
-
-`server/native_transport` stays C++ and owns only GNS listen/connection/message mechanics and endpoint lookup. C# loads the existing C ABI directly.
-
-## Qt Host GUI
-
-The Qt Host remains native C++. It launches the published `CommonwealthOnline.Server` process and uses the authenticated localhost admin channel for stats, clients, kicks and bans.
-
-Build requirements on Windows:
-
-- Visual Studio 2022 C++ tools
-- CMake 3.20+
-- Qt 6.4+
-- .NET 8 SDK
-
-```bat
-check-setup.bat
-build.bat
-deploy.bat
+```bash
+dotnet run --project host/CommonwealthOnline.Host.csproj
 ```
 
-CMake publishes the C# server self-contained and stages it under the Host GUI `server` directory. Packaged users do not need a separate .NET runtime.
+## Build
 
-## Default ports
+```bash
+dotnet build server/CommonwealthOnline.Server.csproj -c Release
+dotnet build host/CommonwealthOnline.Host.csproj -c Release
+```
 
-- TCP 7777: gameplay compatibility
-- UDP 7777: GNS gameplay when enabled
-- UDP 7778: LAN discovery
-- TCP 127.0.0.1:7779: authenticated admin control
-
-See [server/README.md](server/README.md), [SETUP.md](SETUP.md), [DEVELOPMENT.md](DEVELOPMENT.md), and [DEPLOYMENT.md](DEPLOYMENT.md).
+> The Qt/C++ host and its CMake build were retired in favour of the Avalonia
+> host. `SETUP.md` / `DEVELOPMENT.md` / `DEPLOYMENT.md` are from the Qt era and
+> are pending a refresh.