fix: proper SIGINT/SIGTERM handlers for graceful resource release (fixes #121)

[dajiaohuang]

Summary

This PR fixes issue #121 by implementing proper signal handling in the BMF C++ media framework engine, ensuring graceful resource release when the process receives termination signals.

Problems with the original implementation:

1. Used unreliable std::signal() instead of POSIX sigaction()
2. Called non-async-signal-safe functions directly from signal handlers (C++ iostreams, STL operations, mutexes)
3. No protection against multiple rapid signals (double Ctrl+C could leave resources leaked)
4. Race condition: signal handlers installed BEFORE graph added to global list, so early signals would crash
5. Global graph list had no thread synchronization
6. Graph destructor did not clean up the graph pointer from global list
7. Only handled SIGINT and SIGTERM, missing SIGHUP
8. g_ptr.clear() in close() would affect ALL graphs, not just the closing one

Changes made:

• Use sigaction() instead of std::signal() for reliable, portable signal handling
• Add SIGHUP support along with SIGINT and SIGTERM
• Block signals during handler execution using sa_mask to prevent nested signal delivery
• Double-signal protection: First signal triggers graceful shutdown; pressing Ctrl+C again forces immediate exit via _exit()
• Async-signal-safe handler: Signal handler only uses safe operations (write(), atomic operations, _exit()); spawns a detached thread for complex cleanup
• Thread-safe global list: Uses mutex to protect g_graphs vector from concurrent access
• Fix registration order: Add graph to global list BEFORE installing signal handlers
• Proper cleanup: Both close() and destructor remove only this graph from the list, not clear all graphs
• Null safety: Check pointers before operating on them
• User feedback: Clear messages printed to stderr indicating shutdown state

Files changed:

• bmf/engine/c_engine/src/graph.cpp

Testing: Press Ctrl+C during graph execution - all nodes and resources should be properly closed. Press Ctrl+C again if stuck to force exit.