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/engine/src/cmd/sketch_modify.rs

@@ -169,7 +169,7 @@ fn split_at(sk: &mut Sketch, ci: usize, ts: &[f64]) -> Result<(Vec<usize>, Vec<u
     let mut pieces = vec![ci];
     let mut push = |sk: &mut Sketch, kind: CurveKind, prefix: &str| -> Result<()> {
         let id = sk.fresh(prefix);
-        sk.curves.push(Curve { id, kind, construction, reversed: false, link: None, centerline: false });
+        sk.curves.push(Curve { id, kind, construction, reversed: false, link: None, centerline: false, fixed: false });
         pieces.push(sk.curves.len() - 1);
         Ok(())
     };
@@ -980,6 +980,7 @@ fn copy_through(
             reversed: if flip { !cu.reversed } else { cu.reversed },
             link: None,
             centerline: false,
+            fixed: false,
         });
         made.push(sk.curves.len() - 1);
     }