lift the shared Centroid helper into ai/Centroid.h

AdvanceExecutor and StandbyExecutor each carried a verbatim copy of the
struct in an anonymous namespace; it now sits next to OrbitMath.h.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GH8ZMRY3vhxxXcaUxBqxkk

src/lib/ecs/system/CMakeLists.txt

@@ -5,6 +5,7 @@ SET(HDRS
     ${CMAKE_CURRENT_SOURCE_DIR}/ai/AttackEvaluator.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ai/AttackExecutor.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ai/BehaviorTargeting.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/ai/Centroid.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ai/DeliverScrapEvaluator.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ai/DeliverScrapExecutor.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ai/RallyEvaluator.h