feat(draw): Arc Continue command; 3-Point default

Arc ribbon default + icon + dropdown order set to 3-Point. New ARC_CONT: tangent-continuing arc from the last line/arc endpoint, pick end only; Ctrl flips direction. Exit anchor recorded on commit (disambiguated by last pick) so chaining continues from the correct end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/modules/draw/mod.rs

@@ -62,7 +62,7 @@ impl CadModule for DrawModule {
                             label: "Arc",
                             icon: arc::ICON,
                             items: arc::DROPDOWN_ITEMS.to_vec(),
-                            default: "ARC",
+                            default: "ARC_3P",
                         },
                         RibbonItem::Dropdown {
                             id: shapes::DROPDOWN_ID,