Add STEP thumbnail support with bundled converter

[Sagigamil]

Adds Windows Explorer thumbnail support for STEP CAD files (.step / .stp).

Implementation details:

• registers .step and .stp thumbnail providers
• falls back to a bundled space-thumbnails-step-converter.exe when Assimp cannot load the STEP file directly
• uses cadrum / OpenCascade to tessellate STEP into a temporary STL, then reuses the existing Assimp + Filament render path
• includes the converter executable in the Windows installer package
• extends the file-thumbnail timeout for STEP conversion work

Validation performed on Windows:

• cargo build -p space-thumbnails-step-converter --releasen- cargo build -p space-thumbnails-cli --releasen- cargo build -p space-thumbnails-windows-dll --release`n- converted and rendered sample NEO.STEP through the CLI
• installed locally and verified Shell thumbnail generation for both .STEP and .stp via IShellItemImageFactory`n
  Note: this removes the earlier FreeCAD dependency by shipping the converter as part of the project artifacts.