docs: design/verilog-a-source-coverage-tracking.md imports non-existent vajax.coverage module

[github-actions]

File: docs/design/verilog-a-source-coverage-tracking.md (Section 8.1)

Problem: The API design section contains a Python snippet that imports from a module that does not exist:

```python
from vajax.coverage import CoverageTracker, CoverageMode
```

Running this import fails:
```
ModuleNotFoundError: No module named 'vajax.coverage'
```

The vajax/coverage module and CoverageTracker/CoverageMode classes have not been implemented. The document is marked "Status: Draft" and describes a planned feature, but there is no indication in the code examples that the API is hypothetical/not yet implemented.

Fix: Either add a note to the code examples stating these are proposed API designs not yet implemented, or add a prominent banner to the document noting that vajax.coverage does not yet exist.