Broker exit request and nix run .#loop for self-updating hosts

- broker: `{"type":"exit"}` answers `exiting`; `sausage run` then stops all
  actors and exits with code 75.
- flake: apps `loop` / `loop-internal` run `nix run --refresh git+ssh://.../surma/sausage -- run <world>`
  in a loop; exit code 0 (/quit) ends the loop, anything else restarts.
- PROTOCOL.md, README.md, test in cli/tests/host.rs.

Not compiled: written by the Sausage agent, which cannot build the native host.

flake.nix

@@ -35,11 +35,32 @@
           '';
           meta.mainProgram = "sausage";
         };
+        # `nix run .#loop -- <world>` runs the newest `sausage run <world>`
+        # from the Gitea repo, again and again: after a crash or a broker
+        # `exit` request it refreshes the flake and starts the host again,
+        # so the world updates itself. `/quit` (exit code 0) ends the loop.
+        # `--internal` (or `.#loop-internal`) uses Gitea's SSH port on the LAN.
+        publicUrl = "git+ssh://containeruser@gitea.surma.technology:2222/surma/sausage";
+        internalUrl = "git+ssh://containeruser@10.0.0.2:2222/surma/sausage";
+        loop = name: url: pkgs.writeShellScriptBin name ''
+          url="${url}"
+          if [ "''${1:-}" = "--internal" ]; then url="${internalUrl}"; shift; fi
+          if [ $# -eq 0 ]; then set -- world; fi
+          while true; do
+            nix run --refresh "$url#default" -- run "$@"
+            code=$?
+            if [ "$code" -eq 0 ]; then exit 0; fi
+            echo "sausage-loop: sausage exited with $code, starting the newest version in 2 s …" >&2
+            sleep 2
+          done
+        '';
       in
       {
         packages.default = sausage;
 
         apps.default = { type = "app"; program = "${sausage}/bin/sausage"; };
+        apps.loop = { type = "app"; program = "${loop "sausage-loop" publicUrl}/bin/sausage-loop"; };
+        apps.loop-internal = { type = "app"; program = "${loop "sausage-loop-internal" internalUrl}/bin/sausage-loop-internal"; };
 
         devShells.default = pkgs.mkShell {
           packages = [ rust pkgs.wasmtime pkgs.wasm-tools ];