Fix silent corruption on malformed 3MF triangle indices

[boinger]

parse3MF stores v1/v2/v3 into a Uint32Array before validating. The array coerces NaN→0 (and wraps negatives), so the isNaN and < 0 checks are unreachable: a missing or malformed index attribute becomes a valid-looking 0 and the triangle silently loads anchored at vertex 0 — a corrupt mesh headed for the slicer instead of the intended "Invalid triangle index in 3MF file" error.

Same spirit as #37's parser hardening. Fix: validate the raw attribute (digits-only, then range-check) before storing. This also rejects values parseInt silently truncates ("3abc" → 3) or reinterprets ("0x10" → 16). Valid files parse identically.

Repro — zip this as 3D/3dmodel.model (plus the usual [Content_Types].xml / _rels/.rels) and load it; note v2 is missing:

<?xml version="1.0"?>
<model unit="millimeter" xmlns="http://schemas.microsoft.com/3dmanufacturing/core/2015/02">
  <resources><object id="1" type="model"><mesh>
    <vertices><vertex x="0" y="0" z="0"/><vertex x="10" y="0" z="0"/><vertex x="0" y="10" z="0"/></vertices>
    <triangles><triangle v1="0" v3="2"/></triangles>
  </mesh></object></resources>
  <build><item objectid="1"/></build>
</model>

• Before: loads without error; the triangle uses vertex 0 where v2 should be.
• After: throws "Invalid triangle index in 3MF file".