MSCO 2.10.0: mod support, ESC no longer quits from the main menu

- MscoMod API (ModApi.cs) and ModLoader: own mods from MSCO\Mods, hooks for menu/game/update/GUI,
  session and player events, Send/OnMessage between players (MsgType.ModMessage)
- Server mods: ModList before Welcome, consent prompt (INSTALL & JOIN), download with SHA-256 check
  into MSCO\Mods\Servers, active only on that server; game start waits for the download
- Mods page in the MSCO menu, console command "mods"
- Server browser: MODS pill, VANILLA/MODDED filters, mod list in the server profile
- PlayMaker ApplicationQuit is ignored in the main menu (ESC used to close the game)
- MODDING.md and examples/HelloMod

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

MSCO.csproj

@@ -94,6 +94,8 @@
     <Compile Include="MenuHelper.cs" />
     <Compile Include="MenuInputBlocker.cs" />
     <Compile Include="MiniJson.cs" />
+    <Compile Include="ModApi.cs" />
+    <Compile Include="ModLoader.cs" />
     <Compile Include="MenuBranding.cs" />
     <Compile Include="MSCO.cs" />
     <Compile Include="MultiplayerManager.cs" />