feat: plinegen + SAB parsing for Solid3D/Region/Body

LwPolyline PLINEGEN=0 (DXF default): emit SegmentedLines with NaN
separators between vertex-to-vertex segments; wire_gpu resets
arc-length to 0 after each NaN so linetype pattern restarts per vertex.
PLINEGEN=1: existing Contour path unchanged (continuous pattern).
WireModel gains plinegen field propagated through tessellate pipeline.

Solid3D/Region/Body: call SabReader::read() when parse_sat() returns
None due to binary SAB format (DWG files / newer AutoCAD exports).

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/update.rs

@@ -1366,6 +1366,7 @@ impl H7CAD {
                                     tangent_geoms: vec![],
                                     key_vertices: vec![],
                                     aabb: crate::scene::WireModel::UNBOUNDED_AABB,
+                                    plinegen: true,
                                 };
                                 previews.push(guide);
                             }