Add memory locking for deterministic PLC execution

[devin-ai-integration]

Summary

Adds mlockall(MCL_CURRENT | MCL_FUTURE) to the PLC cycle thread initialization to prevent page faults during scan cycle execution. This complements the existing set_realtime_priority() call by ensuring memory pages are not swapped out, eliminating unpredictable latency spikes.

Changes:

• New lock_memory() function in utils.c that calls mlockall()
• Called immediately after set_realtime_priority() in plc_cycle_thread()
• Logs success/failure but continues execution if locking fails (non-fatal)

Note: This requires the container to have --ulimit memlock=-1 set. A companion PR in orchestrator-agent adds this ulimit to vPLC container creation.

Review & Testing Checklist for Human

• Verify compilation: Build the project to ensure <sys/mman.h> include and mlockall() call compile correctly on all target architectures
• Test with proper ulimits: Run container with --ulimit memlock=-1 and verify "Memory locked successfully" appears in logs
• Test without ulimits: Run container without memlock ulimit and verify it logs an error but continues running (graceful degradation)
• Memory impact on constrained devices: Consider if MCL_FUTURE could cause issues on Raspberry Pi or other low-memory devices where locking all future allocations might exhaust physical memory

Recommended test plan:

1. Build Docker image with these changes
2. Run with: docker run --cap-add=SYS_NICE --ulimit rtprio=99 --ulimit memlock=-1 ...
3. Start PLC and check logs for "Memory locked successfully"
4. Monitor memory usage during extended operation

Notes

• The <sys/mman.h> include appears in both utils.h and utils.c - this is slightly redundant but harmless
• This change was not locally compiled/tested before submission

Link to Devin run: https://app.devin.ai/sessions/4a976ea543434bc1b5ee2caf9c8a093e
Requested by: Thiago Alves (@thiagoralves)

[devin-ai-integration]

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