Surface Extend: a planar surface's straight edges pushed out (FusionExtendCommand)

Kernel extend(body, edges, distance) moves the picked straight edges of a
planar surface outward in its plane, with neighbours meeting them. Doc
feature SurfaceExtend, engine FusionExtendCommand. Tests: 10x10 extended
by 5 past one edge gives 150, past two gives 225; the engine patch, trim,
extend and thicken gives 240.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/doc/src/assembly.rs

@@ -261,7 +261,7 @@ impl FeatureKind {
             | FeatureKind::Thicken { .. }
             | FeatureKind::Import { .. }
             | FeatureKind::MeshImport { .. } => {}
-            FeatureKind::Patch { edges, .. } => edges.iter_mut().for_each(pt),
+            FeatureKind::Patch { edges, .. } | FeatureKind::SurfaceExtend { edges, .. } => edges.iter_mut().for_each(pt),
             FeatureKind::SurfaceTrim { plane, tool, keep, .. } => {
                 map_plane_ref(m, plane);
                 if let Some(t) = tool {