[TASK][Timpani-n] Handle SIGTERM and SIGINT signals for shutdown

[LGSI-KarumuriHari]

📝 Task Description

Implement SIGALRM handling, task signal delivery, and shutdown signal processing.

📋 Checklist

• Register signal handlers for SIGTERM, SIGINT, SIGALRM
• Implement graceful shutdown on SIGTERM/SIGINT
• Handle SIGALRM for timer expiration
• Use signal-safe functions only in handlers
• Set shutdown flag atomically
• Coordinate shutdown sequence

🔗 Related Requirement

Relates to #23 (Signal Handling)
Relates to #15 (Implement Signal Handling)

📐 Implementation Guidelines

• Functional Requirement: FR-7.3.1 - Handle SIGTERM and SIGINT signals
• Component: timpani-n/Execution Layer/Signal Handler
• Use sigaction() for signal handler registration
• Handle SIGTERM/SIGINT for graceful shutdown
• Handle SIGALRM for task activation
• Use atomic variables for signal flags

🧪 Testing Method

• Unit test: Verify signal handler registration
• Integration test: Test shutdown via signals
• Safety test: Ensure signal handlers are signal-safe

[jayakrishnan04b]

@basheerFZ Please assign this feature implementation to me.