Invalid IRI accepted

**Summary:** Parser accepts bare email address as IRI object, violating RDF 1.1 specification.

**Input:**
```turtle
<http://aa> <http://aa> <bob@example.com> .
```

**Expected behavior:** Parser should reject `<bob@example.com>` as invalid IRI. Per RFC 3987, an IRI requires a scheme component. Email addresses without `mailto:` scheme do not conform.
