fix(import): geom default resolution, GLB normals, degenerate-transform guards

[jonathanembleyriches]

Problem

Three related import/render defects, surfaced by issue #76 and by re-importing the G1:

1. A geom with no type attribute imports as the base UMjGeom (no renderer) instead of MJCF's global default, sphere. plane, ellipsoid and sdf were never mapped to their component classes. The G1's foot-contact geoms are all typeless.
2. Primitive importers compute RelativeScale3D from size with a zero fallback, so a geom whose size is inherited from a <default> class bakes scale (0,0,0) into the Blueprint template. This is the source of the Scale3D is (nearly) zero physics warnings in [Bug] MjPDController attached to G1 articulation but robot remains fully unresponsive #76 and of InverseFast NIL matrix render spam once snapshot-driven transforms re-dirty the primitive every frame.
3. clean_meshes.py exported GLBs without a NORMAL accessor, and fix_normals() silently degraded when networkx was missing (networkx was never in the plugin's Python dependency list). Every mesh then imported with degenerate tangent bases / nearly zero normals / Bad MeshDescription. Before the June 13 guard (b7a683b) the missing dep crashed the conversion, so imports fell back to the raw source mesh and looked clean, which is why this appeared to be a recent regression.

Changes

• Parser: typeless geoms default to sphere (exempt inside <default> blocks, where forcing a type would clobber inheritance); plane/ellipsoid/sdf mapped; size resolved through the default-class chain for the editor visual while bOverride_size stays false so compile-time inheritance is untouched.
• Primitives: new SyncEditorScaleFromSize() on sphere/cylinder/capsule/box; never writes a non-positive scale.
• Render: MjBody::ApplyRenderState, MjGeom::UpdateGlobalTransform and MjQuickConvertComponent skip zero/NaN snapshot rows and warn once naming the component, instead of flooding the renderer anonymously.
• Mesh pipeline: GLB export passes include_normals=True; networkx added to the dependency check/install; a failed conversion removes its stale GLB (the importer prefers .glb over the source mesh); GLBs older than the script regenerate, so previously broken GLBs self-heal on the next import.

Testing

=== URLab build+test summary ===
Timestamp : 2026-07-16 20:18:49 UTC
Git HEAD : ffffebc6 (fix/geom-scale-and-mesh-normals)
Engine : UE_5.7
Deps : mj=67a1ea6 coacd=c7436bf zmq=7d95ac0
Build : Succeeded
Tests : 269 / 269 passed (0 failed) [269 tests performed]
Log sha256: 77384db7db6ada17
================================

3 new importer tests (typeless geom is a sphere, class-inherited size keeps a non-zero editor scale with no explicit override, plane maps to UMjPlane). Verified live: G1 re-import produces no Bad MeshDescription or zero-normal warnings and PIE runs without InverseFast errors; the companion bridge fix (URLab-Sim/URLab_Bridge#3) was validated end to end against this branch with BeyondMimic on the G1.