Add DWG import fixtures

Author: learncold

tests/fixtures/import/dwg/README.md

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+# DWG Import Fixtures
+
+DWG samples for manual and automated import testing.
+
+These files are kept separate from `tests/dxf/` because they require DWG-capable tooling or conversion before the current DXF import path can consume them directly.
+
+## Included Files
+
+| Path | Source | Notes |
+| --- | --- | --- |
+| `autodesk_acad_samples/*.dwg` | Autodesk AutoCAD LT sample files | Official AutoCAD LT 2010+ sample set. Useful for broad DWG parser/conversion smoke tests. |
+| `cadforum/sample_floor_plan.dwg` | CADForum block catalog | Small residential sample floor plan. |
+| `dwgshare/sample_apartment_floor_plans.dwg` | DWGShare | Apartment floor plan sample extracted from the downloaded ZIP. |
+| `dwgshare/shopping_center_free_autocad_drawings.dwg` | DWGShare | Shopping center plan sample extracted from the downloaded ZIP; useful for evacuation-scale import trials. |
+
+## Provenance
+
+- Autodesk source: https://www.autodesk.com/support/technical/article/caas/tsarticles/ts/01em4r6LLJgnQQVBlk5GqD.html
+- CADForum source: https://www.cadforum.cz/catalog_en/block.asp?blk=12286
+- DWGShare apartment source: https://dwgshare.com/256-free-download-of-cad-drawings-for-sample-apartment-floor-plans/
+- DWGShare shopping center source: https://dwgshare.com/shopping-center-free-autocad-drawings/
+
+## Notes
+
+- The original ZIP archives downloaded from DWGShare are not committed; only the extracted DWG files are kept.
+- AutoCAD can export these DWG files to DXF for the existing import module test path.
+- Keep additional large or third-party CAD files in this folder with provenance notes.