Add telemetry system: protobuf over Unix socket

- TelemetrySocket: non-blocking Unix domain socket writer (fire-and-forget)
- TelemetryEmitter: high-level event emitter with protobuf serialization
- Hooks into: World::update (snapshots), World::onGo (race start),
  ServerLobby (race finish, player join/leave),
  LinearWorld::newLap (laps, kart finish),
  NetworkItemManager (item collection, switch),
  Flyable (projectile hits)
- CMake: finds protobuf, generates C++ from proto schema, links libprotobuf
- Configurable via STK_TELEMETRY_SOCKET env (disabled if unset)
- Snapshot rate configurable via STK_TELEMETRY_SNAPSHOT_HZ (default 10)

CMakeLists.txt

@@ -530,6 +530,35 @@ endif()
 # Provides list of source and header files (STK_SOURCES and STK_HEADERS)
 include(sources.cmake)
 
+# --- Telemetry (protobuf) ---------------------------------------------------
+# Generate C++ code from the telemetry proto file. Only needed for server builds;
+# the telemetry emitter is compiled in unconditionally but is a runtime no-op
+# unless STK_TELEMETRY_SOCKET is set.
+find_package(Protobuf REQUIRED)
+set(TELEMETRY_PROTO "${CMAKE_CURRENT_SOURCE_DIR}/../proto/telemetry.proto")
+# If building inside Docker, the proto file is copied to the source tree.
+if(NOT EXISTS "${TELEMETRY_PROTO}")
+    set(TELEMETRY_PROTO "${CMAKE_CURRENT_SOURCE_DIR}/proto/telemetry.proto")
+endif()
+set(TELEMETRY_PROTO_OUT "${CMAKE_CURRENT_BINARY_DIR}/telemetry_proto")
+file(MAKE_DIRECTORY ${TELEMETRY_PROTO_OUT})
+set(TELEMETRY_PB_CC "${TELEMETRY_PROTO_OUT}/telemetry.pb.cc")
+set(TELEMETRY_PB_H  "${TELEMETRY_PROTO_OUT}/telemetry.pb.h")
+add_custom_command(
+    OUTPUT ${TELEMETRY_PB_CC} ${TELEMETRY_PB_H}
+    COMMAND protobuf::protoc
+        --cpp_out=${TELEMETRY_PROTO_OUT}
+        --proto_path=${CMAKE_CURRENT_SOURCE_DIR}/../proto
+        --proto_path=${CMAKE_CURRENT_SOURCE_DIR}/proto
+        telemetry.proto
+    DEPENDS ${TELEMETRY_PROTO}
+    COMMENT "Generating telemetry protobuf C++"
+)
+set(STK_SOURCES ${STK_SOURCES} ${TELEMETRY_PB_CC})
+set(STK_HEADERS ${STK_HEADERS} ${TELEMETRY_PB_H})
+include_directories(${TELEMETRY_PROTO_OUT})
+include_directories(${Protobuf_INCLUDE_DIRS})
+
 if (USE_APPLE_NETWORK_LIBRARIES)
     add_definitions(-DAPPLE_NETWORK_LIBRARIES)
     set(STK_SOURCES
@@ -703,6 +732,7 @@ target_link_libraries(supertuxkart
     stkirrlicht
     ${Angelscript_LIBRARIES}
     ${MCPP_LIBRARY}
+    ${Protobuf_LIBRARIES}
     )
 
 if (USE_APPLE_NETWORK_LIBRARIES)