+ Add an opaque base-cap backstop so a 3D section cut can NEVER read see-through, even on a wall the per-layer reconstruction cannot resolve. For each multi-material entity the builder also emits its full closed cross-section (the watertight union of the bands always closes, so this needs no interface stitching), carried in a new `Drawing2D.layerBaseCutPolygons` that ONLY the 3D section overlay consumes (the flat 2D drawing, SVG export, and measure/snap paths are untouched). The overlay draws this opaque base first and the per-layer colours over it, so the colours show where they reconstruct and solid cut material shows everywhere else.
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