Fix #128: Add Document.parseFragment() for XML fragment parsing

[gnodet]

Summary

• Add Document.parseFragment(String) static method that parses XML fragments into a List<Node>
• Supports fragments with multiple root-level elements, comments, processing instructions, and text nodes
• Add 10 tests covering various fragment patterns including round-trip preservation

Test plan

• Parse fragment with multiple elements
• Parse fragment with comments, PIs, and trailing text
• Round-trip preservation of fragment formatting
• Null and empty input handling
• Equivalence with Document.of(xml).nodes().toList()

Closes #128

[github-actions]

✅ CI Build PASSED

Build Status: PASSED
Java Version: 21
Maven Command: ./mvnw verify

🎉 All tests passed! The build is ready for merge.