utils: move time conversion and establish requirement traceability

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.

Start with the active time_conversion implementation, its unit test, and the canonical Bazel target under score/datetime_converter. Trace consumers including the OS Mqueue utility and coverage target, then review #395's Utils requirement baseline and the existing C++ gtest traceability metadata. Done means the implementation and test live under score/utils, old targets and headers forward compatibly, and one valid Utils requirement owns only genuinely verifying tests.

Written by the indexing model from the issue text.