Share online membership and game setup with the server

Profile, member join/leave and building a fresh game move into the sim library,
and the packet reader and writer into protocol.h, so the server can run lobbies
exactly as clients do.

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

src/protocol.h

@@ -22,8 +22,10 @@
 //
 // A signal's payload is either a description {sdp, kind} or a candidate {candidate, mid}.
 
+#include <cstring>
 #include <string>
 #include <string_view>
+#include <type_traits>
 
 constexpr int PROTOCOL_VERSION = 2;
 constexpr int LOBBY_MEMBERS_MAX = 4;
@@ -36,6 +38,35 @@ constexpr int KICK_REASON_MAX = 60;
 constexpr const char SERVER_URL_DEFAULT[] = "wss://shweetz.net/smack-a-brother/ws";
 constexpr int SERVER_URL_LENGTH = 128;
 
+// Packets are plain values one after another, as they sit in memory (every platform agrees on these layouts).
+struct PacketWriter
+{
+    std::string bytes;
+
+    template <class T> PacketWriter& put(const T& value)
+    {
+        static_assert(std::is_trivially_copyable_v<T>);
+        bytes.append(reinterpret_cast<const char*>(&value), sizeof(T));
+        return *this;
+    }
+};
+
+struct PacketReader
+{
+    const std::string& bytes;
+    size_t at = 0;
+
+    template <class T> bool get(T& value)
+    {
+        static_assert(std::is_trivially_copyable_v<T>);
+        if (at + sizeof(T) > bytes.size())
+            return false;
+        std::memcpy(static_cast<void*>(&value), bytes.data() + at, sizeof(T));
+        at += sizeof(T);
+        return true;
+    }
+};
+
 // Text from elsewhere (another player, or the server) made safe to show: only printable ASCII (what the game's font
 // has; anything else, emoji included, is dropped), cut to max characters, with spaces at its ends trimmed.
 inline std::string text_clean(std::string_view raw, size_t max)