[Domain] implement DXF happy-path import to canonical geometry

[learncold]

Summary

• add a DXF happy-path import service that parses ASCII DXF entities and normalizes them into canonical geometry
• preserve block child trace metadata, populate top-level trace refs, and extend import issue codes for DXF import failures
• add DXF importer tests for happy-path geometry extraction, missing block definitions, and classic POLYLINE support

Related Issue

• Closes Task-implement DXF happy-path import to canonical geometry #54

Area

• Engine
• Domain
• Application
• Docs
• Build
• Analysis
• Chore

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• ctest --preset test-debug
• Not run (reason below)

Risks / Follow-up

• the importer is still limited to a documented DXF happy-path subset and does not yet cover general IFC or broader CAD variants