Add glTF binary (.glb) export format, reusing the viewer's existing encoder

[pzfreo]

From a comparison with Casys-AI/mcp-build123d: that server exports GLTF binary alongside STEP/STL, specifically so MCP apps with embedded UI can hand the result to a three.js viewer for interactive orbit/pan/zoom inspection.

export() currently supports step, stl, 3mf, dxf, svg (src/build123d_mcp/tools/export.py). glTF/GLB is the natural web-viewer interchange format and would let users drop exported parts straight into three.js, <model-viewer>, Babylon, Blender, etc.

This is cheaper than it looks: src/build123d_mcp/viewer.py already contains a self-contained, zero-dependency glTF 2.0 binary encoder (encode_glb()) used by the live session viewer, fed by the existing tessellation path. Adding glb to _VALID_FORMATS and wiring the tessellate → encode_glb → temp-and-rename write path into export_file() should need no new dependencies.

Suggested scope:

• export(filename, format="glb") for 3D shapes (2D shapes rejected, same as step/stl/3mf)
• Reuse the existing tessellation quality settings the viewer uses, or expose a coarse/fine knob if trivial
• Multi-format export (format="step,glb") should work since it goes through the same list handling