feat: multicolor painted output based on displacement height

[birnam]

Multicolor 3MF painting (grayscale → color mapping)

Summary

Adds a "Multicolor Painting" export mode that maps the displacement texture's grayscale value to any number of user-defined colors, so a single texture can drive slicer AMS / multi-toolhead filament assignment on export. Each color owns a half-open range from its slider's percentage up to the next color's; 0% is black, 100% is white.

What's new

• UI (index.html, style.css, js/main.js): an enable checkbox, a base-color picker, and a dynamic list of color-stop rows (color swatch + 0–100% slider), each addable/removable. Fully undo/redo-aware (multicolorStops is replaced, never mutated, per row change) and persisted in .bumpmesh projects and sessionStorage.
• Live preview (js/previewMaterial.js): the fragment shader samples a small gradient-ramp texture (built from the current stops) using the same raw grayscale value used for export, and uses it as the lit surface color — so what you see in the viewer matches the exported result. Excluded/masked regions correctly fall back to the base color/teal shading instead of showing ramp color.
• Export (js/multicolorPainting.js, js/exporter.js): classifies each triangle (average of its 3 vertex grayscale samples) into a material index and writes a 3MF with per-triangle slic3rpe:mmu_segmentation paint data — the same attribute PrusaSlicer, BambuStudio, OrcaSlicer, and Snapmaker Orca all read, verified against real-world encoders. Supports up to 15 color stops + base (OrcaSlicer's EnforcerBlockerType ceiling).
• Decimation quality (js/decimation.js, js/exportPipeline.js, js/meshRepair.js): decimation gained an optional, opt-in color-boundary guard — when multicolor export is active, it never collapses an edge whose endpoints sit on opposite sides of a color threshold, so paint boundaries stay aligned to real geometry instead of drifting into a decimated triangle's interior. Fully inert when multicolor is off (default decimation path/signature unaffected).
• README: documents the new export option and module.

Testing

• test-multicolor-painting.html — exercises triangle classification and the exported slic3rpe:mmu_segmentation values end-to-end.
• test-decimation-color-boundary.html — confirms the decimation guard never merges across a color threshold while still reducing triangle count within a color region, and confirms the default (non-multicolor) decimation path is byte-for-byte unaffected.
• Manually verified live-preview color accuracy and ramp persistence across displacement-preview toggling, and re-tested after merging in current main (STEP import, preserve-untextured decimation, etc. — no regressions, all conflicts resolved to keep both feature sets working together).

Examples