ARC: a number typed at the end point is the arc length

After Center and Start, typing a length now draws the arc that long along
the circle, running towards the side of the start point the pointer is on.
A length of a full circle or more keeps the prompt.

Closes #1408

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/modules/draw/draw/arc.rs

@@ -1698,6 +1698,9 @@ pub struct ArcCommand {
     step: ArcStep,
     cw: bool,
     plane: WorkingPlane,
+    /// Last pointer position at the end step: a typed arc length runs from
+    /// the start point towards it.
+    cursor: Option<DVec3>,
 }
 
 impl ArcCommand {
@@ -1706,6 +1709,7 @@ impl ArcCommand {
             step: ArcStep::Start,
             cw: false,
             plane: WorkingPlane::default(),
+            cursor: None,
         }
     }
 
@@ -1754,6 +1758,24 @@ impl ArcCommand {
         })
     }
 
+    /// Arc about `c` from `s` whose length along the circle is `len`, running
+    /// towards the side of the start point the pointer is on.
+    fn sc_arc_length(&self, s: DVec3, c: DVec3, len: f64) -> Option<(DVec3, f64, f64, f64)> {
+        let r = c.distance(s);
+        if r < 1e-9 || len <= 0.0 || len >= TAU * r {
+            return None;
+        }
+        let toward = self
+            .cursor
+            .map(|cursor| (angle_xy(c, cursor, self.plane) - angle_xy(c, s, self.plane)).rem_euclid(TAU));
+        let ccw = match toward {
+            Some(span) if span > 1e-9 => span <= std::f64::consts::PI,
+            _ => !self.cw,
+        };
+        let span = len / r;
+        self.sc_angle(s, c, if ccw { span } else { -span })
+    }
+
     /// Sweep from `s` to the cursor about `c`, going the way Ctrl says.
     fn cursor_span(&self, s: DVec3, c: DVec3, cursor: DVec3) -> f64 {
         let span = (angle_xy(c, cursor, self.plane) - angle_xy(c, s, self.plane)).rem_euclid(TAU);
@@ -1960,7 +1982,11 @@ impl CadCommand for ArcCommand {
                     self.step = ArcStep::LengthSC { s, c };
                     Some(CmdResult::NeedPoint)
                 }
-                _ => None,
+                // A bare number is the arc's length along the circle (#1408).
+                _ => {
+                    let len = Self::typed_number(text)?;
+                    Some(self.commit(self.sc_arc_length(s, c, len)))
+                }
             },
             ArcStep::CenterSE { s, e } => match upper.as_str() {
                 "A" | "ANGLE" => {
@@ -2135,10 +2161,12 @@ impl CadCommand for ArcCommand {
                     .into_iter()
                     .collect()
             }
-            ArcStep::EndSC { s, c } => self
+            ArcStep::EndSC { s, c } => {
-                .preview(self.sc_end(s, c, pt), line_wire(c, pt))
+                self.cursor = Some(pt);
-                .into_iter()
+                self.preview(self.sc_end(s, c, pt), line_wire(c, pt))
-                .collect(),
+                    .into_iter()
+                    .collect()
+            }
             ArcStep::AngleSC { s, c } => {
                 let span = self.cursor_span(s, c, pt);
                 self.preview(self.sc_angle(s, c, span), line_wire(c, pt))