perf: 200-robot simulation optimization — parallel plugin loop + -O3/LTO

[sabarish-prasannna]

Summary

Builds on feature/90robots-physics-skip. Adds two complementary optimizations to bring 200-robot simulation CPU from ~90% down to <20%:

• Parallel plugin loop: PluginManager::BeforePhysicsStep and AfterPhysicsStep now dispatch each robot's plugin group (SootballNavigatorPlugin + SootballPlugin) as an independent std::async task, utilizing all available CPU cores. World plugins remain sequential after all model plugins complete.
• -O3 + LTO: flatland_lib and flatland_server now compile with -O3 -march=native and link-time optimization (skipped when COVERAGE=ON).

Details

Parallel plugin dispatch (plugin_manager.cpp)

• Groups model_plugins_ by Model* pointer — each robot owns exactly its N plugins.
• Launches one std::future<void> per robot, waits on all before proceeding to world plugins.
• Thread-safe: Box2D body reads outside physics step are safe, each robot writes only its own bodies, ros::Publisher::publish() is thread-safe in roscpp.
• PROFILER macros kept outside lambdas to avoid concurrent map access.

Build flags (CMakeLists.txt)

if(NOT "${COVERAGE}" STREQUAL "ON")
    target_compile_options(flatland_lib PRIVATE -O3 -march=native)
    target_compile_options(flatland_server PRIVATE -O3 -march=native)
    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.9)
        set_property(TARGET flatland_lib PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
        set_property(TARGET flatland_server PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
    endif()
endif()

Expected Impact

Change	Gain
Parallel plugin loop	4–8× on 8–16 core machine
-O3 + LTO	1.2–1.4× raw throughput

Test plan

• Build: catkin build flatland_server
• Run 90-robot fast sim, confirm CPU drop vs feature/90robots-physics-skip baseline
• Run 200-robot fast sim, confirm CPU < 20%
• Confirm robots still navigate correctly (no stuck robots, task completion rate unchanged)
• Check /tmp/flatland_profile_output.log — "Before Physics Step: model_plugins (parallel)" time should drop proportionally

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