Summary

• Adds a new Scan Mode option to the raster engraver with two choices: Segmented (default, existing behavior) and Full Sweep (new)
• Full Sweep scans the entire width of each scan line with laser power toggling on/off, instead of splitting into separate segments per content region. This dramatically reduces acceleration/deceleration moves for high-contrast images with many separate black regions.
• Works across all depth modes: Power Modulation, Constant Power, Dither, and Multi-Pass
• Includes UI toggle in the Engraving Pattern settings group, serialization with backward compatibility, and comprehensive tests

Closes #253

Context

When engraving images with high contrast (many separate black regions per line), the segmented approach causes the laser to accel/decel for each region, resulting in 4x+ longer engraving times. Full Sweep trades slightly reduced sharpness (laser passes over white regions at zero power) for significantly faster engraving.