Fix/Unfix on every sketch entity: points (Fix constraint) and lines, arcs, circles, ellipses, splines and conics (fixed curves hold their points and radius); ConstraintFix takes entities (fixes them all unless all are fixed); the toolbar button and the context menu act on the whole selection; fixed points never move when dragged or solved; projected geometry counts as fixed; fixed geometry is drawn green; sketch.inspect reports fixed

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

crates/sketch/src/solver.rs

@@ -70,6 +70,13 @@ fn layout(sk: &Sketch) -> Layout {
             *f = true;
         }
     }
+    for c in sk.curves.iter().filter(|c| c.fixed) {
+        for p in c.kind.point_ids() {
+            if let Some(f) = fixed.get_mut(p) {
+                *f = true;
+            }
+        }
+    }
     let mut n = 0;
     let pvar = fixed
         .iter()
@@ -86,7 +93,7 @@ fn layout(sk: &Sketch) -> Layout {
         .curves
         .iter()
         .map(|c| match c.kind {
-            CurveKind::Circle { .. } | CurveKind::Ellipse { .. } if c.link.is_none() => {
+            CurveKind::Circle { .. } | CurveKind::Ellipse { .. } if c.link.is_none() && !c.fixed => {
                 n += 1;
                 Some(n - 1)
             }