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/params.rs

@@ -226,6 +226,7 @@ impl FeatureKind {
             FeatureKind::Stitch { tolerance, .. } => v.push(("Tolerance", tolerance, L)),
             FeatureKind::Thicken { thickness, .. } => v.push(("Thickness", thickness, L)),
             FeatureKind::SurfaceTrim { plane, .. } => plane_inputs_mut(plane, &mut v),
+            FeatureKind::SurfaceExtend { distance, .. } => v.push(("Distance", distance, L)),
             FeatureKind::Patch { .. } => {}
             FeatureKind::Combine { .. }
             | FeatureKind::SheetBase { .. }