Fix crash when slicing 3MF files without vertex normals

[Copilot]

3MF files from MakerWorld (and similar sources) omit vertex normals from geometry data. The Polytree library's fromMesh() unconditionally accesses geometry.attributes.normal.array, throwing TypeError: Cannot read properties of undefined (reading 'array') when the attribute is absent.

Changes

• src/slicer/slice.coffee — After cloning the mesh geometry, call computeVertexNormals() when the normal attribute is missing. Operates only on the internal clone; original mesh is unmodified.

# Ensure geometry has vertex normals before slicing.
# Some file formats (e.g., 3MF from MakerWorld) do not include normal data.
# Polytree.fromMesh requires normals to convert mesh geometry into polygons.
if not mesh.geometry.attributes.normal
    mesh.geometry.computeVertexNormals()

• src/slicer/slice.test.coffee — Adds a Mesh Without Normals test that constructs a BufferGeometry with position data only (no normals), confirms slicing completes without throwing, produces valid G-code, and leaves the original geometry unmodified.

[Copilot]

Pull request overview

This PR prevents slicing from crashing on meshes whose geometry omits vertex normals (common in some 3MF exports) by computing normals on the slicer’s internal geometry clone, and adds a regression test to ensure slicing succeeds without mutating the original input geometry.

Changes:

• Compute vertex normals on the cloned mesh geometry when the normal attribute is missing before invoking Polytree slicing.
• Add a new test case covering slicing a mesh that has position data but no vertex normals.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/slicer/slice.coffee	Computes missing vertex normals on the cloned geometry to avoid Polytree slicing crashes.
src/slicer/slice.test.coffee	Adds a regression test ensuring slicing succeeds when normals are absent and the original geometry remains unmodified.