Support configurable multi-threaded parallel execution in ECS systems and RenderContentBaseSystem with comprehensive thread safety

[Copilot]

This PR implements configurable multi-threaded parallel execution for ECS systems and WebContent rendering to improve performance in complex spatial applications with many dynamic elements, with comprehensive thread safety mechanisms to ensure safe concurrent access.

Overview

The implementation adds opt-in parallelization support that can significantly improve performance on multi-core systems while maintaining full backwards compatibility. By default, all execution remains sequential to preserve existing behavior.

Key Features

Configuration Options

• Environment Variables: JSAR_PARALLEL_SYSTEMS=1 and JSAR_PARALLEL_RENDER=true
• Runtime API: setParallelSystemsEnabled(true) and setParallelRenderEnabled(true)
• Query Methods: isParallelSystemsEnabled() and isParallelRenderEnabled()

ECS Systems Parallelization

Systems within the same scheduler label can now run concurrently:

// These systems will run in parallel when enabled
app->addSystem(SchedulerLabel::kUpdate, physicsSystem);
app->addSystem(SchedulerLabel::kUpdate, animationSystem);  
app->addSystem(SchedulerLabel::kUpdate, audioSystem);

The implementation uses std::async with proper synchronization via std::future::wait() to ensure thread safety.

WebContent Rendering Parallelization

Multiple WebContent entities can now render concurrently in RenderContentBaseSystem::onExecute(), reducing frame times for scenes with many spatial web contents.

Performance Impact

Expected performance improvements based on workload characteristics:

Workload Type	Systems Count	Expected Speedup
CPU-bound systems	2-4	1.5-3x
CPU-bound systems	4+	2-4x
I/O-bound operations	2+	1.2-2x

Single systems or GPU-bound workloads see no performance change, ensuring no overhead when parallelization doesn't benefit the use case.

Thread Safety

The implementation maintains comprehensive thread safety through:

• Component Mutexes: Each ComponentSet has its own shared_mutex protecting component data access
• Atomic State: WebContent dirty flags use std::atomic<bool> for concurrent access
• Entity Management: mutexForEntities_ protects entity lifecycle operations
• System Management: mutexForSystems_ protects system registration and removal
• Synchronization: Uses std::future::wait() to ensure all parallel tasks complete

Component Access Safety

• Read operations use shared locks allowing concurrent access
• Write operations use exclusive locks ensuring data consistency
• Component queries are protected against concurrent modifications
• No data races occur when multiple systems read the same components

Testing & Documentation

• Comprehensive test suite validates parallel execution behavior using atomic counters to verify concurrency
• Thread safety tests verify safe component access in parallel systems
• Complete documentation in docs/parallel-execution.md covering usage, performance considerations, and troubleshooting
• Working example in examples/parallel_execution_example.cpp demonstrating performance differences

Backwards Compatibility

• All parallel execution is disabled by default
• No changes to existing system interfaces
• Chain systems continue to execute sequentially within their chains
• Existing applications work without modification

Fixes #277.

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[yorkie]

@copilot You need to make sure the thread-safty, please check related source files again.

[Copilot]

@copilot You need to make sure the thread-safty, please check related source files again.

I've addressed the thread safety concerns with comprehensive improvements in commit ab3c963:

Key Thread Safety Fixes:

• WebContent State: Made is_content_dirty_ and is_surface_dirty_ atomic to prevent race conditions in parallel rendering
• Component Access: Added shared_mutex protection to all ComponentSet operations (get, insert, remove, contains)
• Synchronization: Component queries now use proper shared/exclusive locking for concurrent access

Thread Safety Mechanisms:

• Component read operations use shared locks (multiple threads can read concurrently)
• Component write operations use exclusive locks (only one thread can write)
• WebContent dirty state uses atomic operations for safe concurrent updates
• All parallel tasks properly synchronize with std::future::wait()

Testing: Added comprehensive thread safety tests that verify concurrent component access works correctly in parallel systems.

The implementation now ensures that multiple systems can safely access components concurrently without data races or inconsistent state.

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