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Determines whether a URL string refers to a cross-origin resource, using purely static string analysis without any `window` or runtime context. Designed to be SSR-deterministic, avoiding React hydration mismatches caused by server/client value divergence.

## Key Components

**`isCrossOriginUrl(url)`**
Returns `true` if the URL would resolve to a different origin; `false` for relative paths, fragments, query-only strings, and nullish values.

| Input pattern | Result | Reason |
|---|---|---|
| `null` / `undefined` / `""` | `false` | No URL |
| `#anchor`, `?tab=x` | `false` | Same-document reference |
| `/path/to/page` | `false` | Relative path, same origin |
| `//cdn.example.com/...` | `true` | Protocol-relative, explicit host |
| `https://flamingo.run/...` | `true` | Absolute URL, explicit host |
| `http://external.com/...` | `true` | Absolute URL, explicit host |

## Usage Example

```typescript
import { isCrossOriginUrl } from './is-cross-origin-url'

// Navigation link — open externally if cross-origin
const href = buildContentURL(item)
const openInNewTab = isCrossOriginUrl(href)

// <a href={href} target={openInNewTab ? '_blank' : undefined} />

isCrossOriginUrl('/support')             // false — same-origin relative path
isCrossOriginUrl('https://flamingo.run') // true  — external absolute URL
isCrossOriginUrl('#section')             // false — fragment, same page
isCrossOriginUrl(null)                   // false — safe nullish handling
```

## Notes

- Intentionally avoids `window.location` comparisons to stay SSR-safe
- Same-platform absolute URLs (e.g. from `buildContentURL`) are expected to be handled upstream via `targetPlatform` checks before this function is reached
- Source: [`packages/utils/is-cross-origin-url.ts`](https://github.com/flamingo-stack/openframe-oss-lib/blob/main/packages/utils/is-cross-origin-url.ts)