Let the conic tool take a typed Rho, and preview the curve it is about to draw (#736)

The conic tool takes a typed Rho in its canvas value box and previews the curve it's about to draw, with the tangent legs as a faint guide.

crates/engine/src/cmd/sketch_ux_tests.rs

@@ -534,6 +534,32 @@ fn dragging_circles_and_arcs() {
     assert!(pt(&s, "a1.start").dist(v(70.0, 0.0)) < 1e-9, "{:?}", pt(&s, "a1.start"));
 }
 
+#[test]
+fn dragging_a_conic_reads_rho_off_the_cursor_and_its_apex_slides_like_any_point() {
+    let mut s = new_sketch();
+    run(&mut s, "sketch.conic", json!({"start": [0, 0], "end": [40, 0], "apex": [20, 20]}));
+    let rho = |s: &Session| match sketch(s).curves[0].kind {
+        solidcraft_sketch::CurveKind::Conic { rho, .. } => rho,
+        ref k => panic!("not a conic: {k:?}"),
+    };
+    assert!((rho(&s) - 0.5).abs() < 1e-9, "default rho: {}", rho(&s));
+
+    // Grabbed on the curve itself (its midpoint, at the default rho of 0.5): `rho` follows the
+    // cursor, read off where it falls between the chord's midpoint (20, 0) and the apex (20,
+    // 20). The three defining points stay exactly where they were.
+    drag(&mut s, "k1", [20.0, 10.0], [20.0, 14.0]);
+    assert!((rho(&s) - 0.7).abs() < 1e-6, "rho after drag: {}", rho(&s));
+    assert!(pt(&s, "k1.start").dist(v(0.0, 0.0)) < 1e-9);
+    assert!(pt(&s, "k1.end").dist(v(40.0, 0.0)) < 1e-9);
+    assert!(pt(&s, "k1.apex").dist(v(20.0, 20.0)) < 1e-9);
+
+    // Grabbed on the apex itself: it slides like any other point (not a `rho` edit), and `rho`
+    // is unaffected.
+    drag(&mut s, "k1.apex", [20.0, 20.0], [10.0, 30.0]);
+    assert!(pt(&s, "k1.apex").dist(v(10.0, 30.0)) < 1e-9);
+    assert!((rho(&s) - 0.7).abs() < 1e-6, "rho after apex drag: {}", rho(&s));
+}
+
 #[test]
 fn constrained_geometry_resists_drags() {
     let mut s = new_sketch();