Fix missing rectangles in STEP output

[mendarb]

Summary

• Root cause: Component boxes (resistors, capacitors, etc.) were converted to triangulated meshes with all triangles placed in a single ClosedShell. This produced invalid B-Rep geometry because multiple disconnected boxes in one shell is not a valid manifold solid, and triangular faces don't share edges properly.
• Fix: Created a proper createBRepBoxSolid function that generates each component as its own ManifoldSolidBrep with 8 vertices, 12 shared edges, and 6 rectangular AdvancedFace entities — a valid closed shell. Boxes with custom meshes (from external models) still use the triangle approach but are now individual solids.
• Result: OCCT now properly renders each resistor/component as a distinct rectangular box instead of dropping them.

Before: 26 separate triangle meshes (each just 1 triangle), components invisible in most viewers
After: 3-4 proper solid meshes (board + individual component boxes), all rendering correctly

Test plan

• All existing tests pass (12/12)
• Added basics07 test: verifies 3 resistor boxes appear as proper solids with correct dimensions (3x1.5x1.5mm)
• Validated via occt-import-js that each component produces a proper mesh with correct bounding box
• bunx biome format --write . applied

Closes #6

/claim #6

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