Fix DXF encoding: convert TV strings to UTF-8

## Summary Fixes encoding issues where LibreDWG was outputting DXF files with: - UTF-8 encoded bytes in text fields - But declaring CP1252 codepage in `$DWGCODEPAGE` header This caused ezdxf to read the UTF-8 bytes as CP1252, creating surrogate characters (`\udcxx`) for undefined bytes in CP1252. ## Changes 1. **dwg_api.c**: Set default codepage to `CP_UTF8` (0) instead of `CP_ANSI_1252` (30) 2. **out_dxf.c**: Convert TV strings from source codepage to UTF-8 before output 3. **out_dxf.c**: Always declare UTF8 codepage in DXF output ## Impact - DXF files will have properly converted UTF-8 text - Compatible with standard DXF readers like ezdxf - No more surrogate character encoding errors