Add a README with screenshots

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

README.md

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+# Smack a Brother
+
+![1](/screenshots/1.png)
+
+![2](/screenshots/2.png)
+
+![3](/screenshots/3.png)
+
+![4](/screenshots/4.png)
+
+Online platform fighter. Lobbies of up to 16 players, games of up to 8. Play it in the browser [here](https://shweetz.net/smack-a-brother/).
+
+## Clarification
+This application was developed with the assistance of a large language model.
+
+## Build
+
+After cloning and entering the repository's directory, make sure to initialize the submodules:
+
+```git submodule update --init --recursive```
+
+Everything (SDL, libdatachannel, simdjson) is built statically from the submodules. The game's resources are in `resources/` and get linked next to the executable.
+
+### Linux
+
+Needs OpenSSL, plus the X11/Wayland and audio headers SDL builds against.
+
+```
+mkdir build
+cd build
+cmake ..
+make
+```
+
+### Web
+
+Needs the [Emscripten SDK](https://emscripten.org/docs/getting_started/downloads.html).
+
+```
+emcmake cmake -B build-web -DCMAKE_BUILD_TYPE=Release
+cmake --build build-web
+```
+
+This makes `smack-a-brother.html`, `.js`, `.wasm` and `.data`. Rename the `.html` to `index.html` and serve all four.
+
+### Server
+
+The server builds on its own, without SDL. It needs OpenSSL (`libssl-dev` on Debian).
+
+```
+cmake -S server -B build-server -DCMAKE_BUILD_TYPE=Release
+cmake --build build-server
+```
+
+Run it with:
+
+```smack-a-brother-server [port] [first UDP port] [last UDP port]```
+
+Defaults are `47800`, `47810` and `47899`. It only listens on `127.0.0.1`, so put it behind a reverse proxy that handles TLS. For nginx:
+
+```
+location /smack-a-brother/ws {
+    proxy_pass http://127.0.0.1:47800;
+    proxy_http_version 1.1;
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection "upgrade";
+    proxy_set_header Host $host;
+    proxy_read_timeout 120s;
+}
+```
+
+Open the UDP range in the firewall (e.g. `ufw allow 47810:47899/udp`).
+
+## Netcode
+
+- The server runs every lobby: a sandbox everyone in the lobby shares, plus the games queued in it. Each is its own simulation. All of them run on one thread, at 60 ticks a second.
+- Clients send their inputs ahead of time. Each tick the server takes what has arrived (repeating a player's last inputs if they're late), steps the simulation, and sends everyone the inputs it used. Every 60 ticks it also sends a checksum.
+- Clients run the same simulation about 3 ticks ahead of the server, guessing other players' inputs. When the server's inputs differ from the guess, the client rolls back and replays.
+- The simulation is deterministic on every platform (strict floating point, own maths functions), so desktop and web play together. `tools/determinism_check.sh` checks this.
+- Clients connect over a WebSocket first (lobbies, chat, WebRTC signaling). Then they open a WebRTC connection to the server with two data channels: a reliable one, and an unreliable one for inputs and frames. If WebRTC can't connect, everything stays on the WebSocket.
+- STUN: `stun.l.google.com:19302`.
+- The server address is set in Settings. Default: `wss://shweetz.net/smack-a-brother/ws`.
+
+### Joining a lobby directly
+- Desktop: `smack-a-brother --join CODE`
+- Web: `https://shweetz.net/smack-a-brother/?lobby=CODE`
+
+### Environment variables
+- `SAB_SERVER_URL`: use a different server, e.g. `ws://127.0.0.1:47800`
+- `SAB_NET_LAG`: add this many milliseconds of lag
+- `SAB_NET_LOSS`: drop this percent of input packets
+- `SAB_NET_STATS`: log how the client is keeping up with the server