utils: move time conversion and establish requirement traceability #556

The full DateTimeConverter implementation was intentionally removed after repeated correctness defects, including leap-second handling. The remaining time_conversion helper is not a DateTime Converter: it only converts chrono durations and absolute timeouts to timespec, and it previously belonged to Utils. Keeping it under score/datetime_converter creates a misleading standalone component boundary with no requirements or documentation.

Move the active time_conversion implementation, unit test, and canonical Bazel target from score/datetime_converter to score/utils. Update in-repository consumers, including the OS Mqueue utility and coverage target, to use the new Utils target. Preserve source and Bazel compatibility at the old location through a deprecated //score/datetime_converter:time_conversion forwarding target and compatible forwarding headers as needed for existing external consumers.

After #395 has established the current Utils requirement baseline, define an atomic comp_req__utils__... requirement for the observable time-conversion behavior, with valid upstream traceability, and allocate it to the Utils architecture. Link the relocated C++ gtests only to that requirement where they genuinely verify it, using mandated FullyVerifies or PartiallyVerifies, TestType, DerivationTechnique, and self-contained Description metadata.

Do not create standalone DateTime Converter requirements. The compatibility target exists only for migration and must not be treated as the owning implementation.