Add IPv6 address extraction

[rolandpg]

Summary

Add IPv6 address extraction to the entity extractor.

Context

The entity extractor handles IPv4 but not IPv6 addresses. CTI reports increasingly reference IPv6 IOCs (e.g., C2 servers, infrastructure).

• File to edit: src/zettelforge/entity_indexer.py
• Tests: tests/test_basic.py::TestEntityExtractor

Acceptance Criteria

• Add ipv6 to REGEX_PATTERNS in entity_indexer.py
• Pattern matches standard IPv6 formats: full (2001:0db8:85a3::8a2e:0370:7334), compressed (::1, fe80::1), and mixed (::ffff:192.168.1.1)
• Add ipv6 to ENTITY_TYPES
• Does NOT false-positive on SHA-256 hashes (hex strings with colons in other contexts)
• At least 4 test cases:
  • Full IPv6: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
  • Compressed: fe80::1
  • Loopback: ::1
  • No false positive on hash: a1b2c3d4e5f6... should not match
• All existing tests pass: pytest tests/test_basic.py -v

Example Input/Output

ext = EntityExtractor()
result = ext.extract_all("C2 server at 2001:db8::1 was observed")
assert "2001:db8::1" in result["ipv6"]