score/time: Create AoU requirements

> [!IMPORTANT]
> Link the implementing PR to this issue by including `Closes #80` in the PR description or commit message.

### What

Write the Assumption of Use requirements for `score/time`, capturing all preconditions that callers of the public API must satisfy.

The `score/time` public API (`Clock`, `ScopedClockOverride`, `ClockTestFactory`) imposes preconditions on callers that must be formally documented. Examples: the backend must be registered before calling `GetInstance()`; `WaitUntilAvailable()` must be called before reading `VehicleTime`; the calling application is responsible for handling `IsAvailable() == false` gracefully. Author as `aou_req` sphinx-needs directives at `score/time/docs/requirements/time_aou.rst`.

### Acceptance Criteria

- [ ] `score/time/docs/requirements/time_aou.rst` created and indexed in component docs
- [ ] All caller preconditions captured as `aou_req` nodes with `:status: valid`
- [ ] Every `aou_req` node links back to the relevant `comp_req` it restricts

### References

- [S-CORE Requirements Engineering](https://eclipse-score.github.io/process_description/main/process_areas/requirements_engineering/)