Add tests for DXF block INSERT and simple line file import

Both BlockXTestFile.dxf and BlockRefXTestFile.dxf are user-reported
files where import was suspected to be incomplete. Confirmed via
console tests that both import correctly:
- BlockXTestFile: 1 Line + 1 Block (2-child INSERT expansion)
- BlockRefXTestFile: 1 Line (VIEWPORT in paper space is correctly skipped)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ka4ocDhmumPuQDD2fDs7ad

Author: claude