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 <[email protected]>

src/app/update/dynamic.rs

@@ -198,6 +198,26 @@ impl OpenCADStudio {
         }
     }
 
+    /// Push the live Ctrl state into tab `i`'s active command before it builds a
+    /// preview/commit (arc-direction flip on `ARC_CONT`, etc.).
+    pub(in crate::app) fn push_ctrl_to_cmd(&mut self, i: usize) {
+        let ctrl = self.ctrl_down;
+        if let Some(c) = self.tabs[i].active_cmd.as_mut() {
+            c.set_ctrl(ctrl);
+        }
+    }
+
+    /// Cache the endpoint + exit tangent of a just-committed line/arc so a
+    /// following `ARC_CONT` starts tangentially from where drawing ended. Set to
+    /// `None` for any other entity kind. Uses `last_point` (the final pick) to
+    /// pick the endpoint the pen actually finished on.
+    pub(in crate::app) fn update_cont_anchor(&mut self, entity: &acadrust::EntityType) {
+        let last = self
+            .last_point
+            .map(|v| glam::DVec3::new(v.x as f64, v.y as f64, v.z as f64));
+        self.cont_anchor = crate::modules::draw::draw::arc::continue_anchor(entity, last);
+    }
+
     pub(in crate::app) fn dyn_resolve_point(&self) -> Option<glam::Vec3> {
         use crate::app::document::DynComponent;
         let i = self.active_tab;

src/app/update/mod.rs

@@ -1751,8 +1751,17 @@ impl OpenCADStudio {
             }
 
             Message::SetModifiers { shift, ctrl } => {
+                let ctrl_changed = self.ctrl_down != ctrl;
                 self.shift_down = shift;
                 self.ctrl_down = ctrl;
+                // A live command may key its preview off Ctrl (arc-direction
+                // flip). Rebuild it at the current cursor so the flip shows
+                // without waiting for the next mouse move.
+                let i = self.active_tab;
+                if ctrl_changed && self.tabs[i].active_cmd.is_some() {
+                    let p = self.tabs[i].last_cursor_screen;
+                    return Task::done(Message::ViewportMove(p));
+                }
                 Task::none()
             }
 

src/app/update/viewport.rs

@@ -1090,9 +1090,11 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                         s.screen.y += tile_b.y;
                     }
 
-                    // Give the command the current UCS before it builds its
-                    // rubber-band preview (and, by persistence, its commit).
+                    // Give the command the current UCS + Ctrl state before it
+                    // builds its rubber-band preview (and, by persistence, its
+                    // commit).
                     self.push_ucs_to_cmd(i);
+                    self.push_ctrl_to_cmd(i);
                     let mut previews = if needs_structure {
                         let mut p = self.tabs[i]
                             .active_cmd