Inspector port conflict when HarperDB is imported multiple times in Next.js

## Problem

When a Next.js app imports `harperdb` (e.g., `import('harperdb')`), Harper's initialization runs multiple times across Next.js build/runtime phases. With thread debugging enabled this causes repeated errors:

```
Starting inspector on 127.0.0.1:9229 failed: address already in use
```

Harper should guard against re-initialization when already running in the same process, or detect/handle the address-in-use case gracefully for the inspector.

🤖 Filed by Claude on behalf of Kris.