Add Line Trace image import algorithm for line drawings

[olavl]

Summary

• Adds a new Line Trace image import algorithm that extracts actual drawn lines from line drawing images, instead of raster-scanning with brightness modulation
• Uses skeleton-tracing-js (Zhang-Suen thinning + polyline extraction) to skeletonize the image into single-pixel-width centerlines, then connects them into a single continuous path via nearest-neighbor ordering
• Exposes three new settings: Threshold (binarization cutoff), Min segment length (noise filter), and Simplify tolerance (Douglas-Peucker smoothing)
• Fixes imageAmplitude option missing isVisible guard (was incorrectly shown for Waves)

Motivation

The existing 6 image import algorithms (Squiggle, Spiral, etc.) all treat the image as a tone map and modulate waveforms based on pixel brightness. For images that are already line drawings, users want the sand table ball to trace the actual lines rather than producing a raster pattern. This new algorithm fills that gap.

How it works

1. pixelProcessor applies brightness/contrast/inversion (existing controls)
2. 3x3 box blur smooths JPEG compression artifacts
3. Threshold to binary (configurable)
4. TraceSkeleton.fromBoolArray() skeletonizes and extracts polylines
5. Short segments filtered (configurable min length)
6. Douglas-Peucker simplification (configurable tolerance)
7. Nearest-neighbor connects all polylines into one continuous path

Test plan

• npm run build succeeds
• All 125 existing tests pass
• Upload a line drawing image, select "Line Trace" from the Type dropdown
• Verify only Threshold/Min segment length/Simplify tolerance appear (plus global brightness/contrast/inversion)
• Adjust threshold — captures more/fewer lines
• Export as GCode and verify single continuous path

🤖 Generated with Claude Code