Add --preview HTML viewer to scrollcase build

[csparker247]

Summary

scrollcase build --preview writes <name>-preview.html next to the output STLs and opens it in the default browser. It's a single file with no server: the parts are embedded as one GLB, and three.js loads from jsDelivr, so viewing it needs a network connection.

The viewer has:

• Per part: show/hide, color picker, and an opacity slider. Each part draws a depth-only pass first, so the stacked honeycomb walls fade smoothly instead of switching between solid and see-through.
• Explode: pulls the halves apart along the split axis (Y) and moves the stand out past the left half.
• Poses: Upright (case +Z up) or On stand (lying on the stand at −Y; labelled On side when there's no stand). The model rotates rather than the camera, so the orbit controls always work in a normal Y-up world.
• View: ground grid toggle, background color, reset view, and Save snapshot, which downloads a PNG of just the 3D view, without the panel.

Upstream villa used the OCP CAD Viewer in VS Code and trimesh's notebook viewer for this; we hadn't ported either.

trimesh moves from a dev dependency to a runtime one. Nothing else is new in pyproject.toml.

Testing

• tests/test_preview.py checks that every part is embedded as a named GLB node in display order, and that the title is escaped in both <title> and the inline JSON.
• Full suite: 69 passed.
• Checked by hand in Chrome on examples/generic.toml (about a 9.7 MB page): opacity fade, explode in both poses, grid toggle, part and background colors, and snapshot output (a valid PNG with no panel). No console errors.

🤖 Generated with Claude Code