[Resolved] 3MF band meshes lost material where slopes cross filament boundaries

[everyday3d]

Bug

3MF band meshes lost material where the surface crosses a filament boundary.

Root cause

Each band mesh kept only quads whose four corners were all above the band floor. A quad that straddles a boundary belonged to no band, so the 3MF solid was shorter than the STL and the heightmap.

Reproduction

Export a 3MF from a photo with slopes across swap layers. On a 96x96 test image 4471 quad-band pairs were dropped, about 3% of the solid volume, and peaks above a band ceiling lost their tops.

Resolution

Bands are now cut exactly against their floor and ceiling planes (src/engine/bandMesh.ts). Intersections are computed from the original triangle edge so parts stay watertight. A test shows the band volumes add up to the whole solid, and the new export verifier rebuilds the solid from the written 3MF and compares it with the heightmap before saving.

Issue: #33. Commit 1d26650.