fix: refine CPU tick timer semantics

[LimiNode]

Summary

• add a CpuTickTimer helper that wraps get_cpu_time() for elapsed, restart, and sample statistics, documenting tick units and invariants
• add stop control and compensated accumulation to the timer
• cover the new timer with unit checks inside the existing time_utils_test, including stop/resume behaviour

Testing

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build

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https://chatgpt.com/codex/tasks/task_e_68d23e667f68832c8ba06a39d0e7e541