feat(network): integrate protobuf for player sync and session management

Add Protobuf dependency, define proto messages for player requests, positions, and chunk data. Refactor Session to use strand and async write. Implement player join/exit and position sync in ServerWorld.

src/gameplay/server_player.cpp

@@ -0,0 +1,10 @@
+#include "Cubed/gameplay/server_player.hpp"
+
+namespace Cubed {
+ServerPlayer::ServerPlayer(std::string_view name) : m_name(name) {}
+const glm::vec3& ServerPlayer::get_pos() const { return m_pos; }
+const std::string& ServerPlayer::get_name() const { return m_name; }
+void ServerPlayer::update_pos(float x, float y, float z) {
+    m_pos = glm::vec3{x, y, z};
+}
+} // namespace Cubed