Spiral tool: Alt-drag adds or removes segments, and Ctrl/Cmd-drag changes the decay

The reference app adds or subtracts winds from a spiral with Alt-drag
and changes its decay rate with Ctrl/Cmd-drag. VectorCraft's Spiral
tool did nothing with either key during a drag.

While a spiral is dragged out, Alt or Ctrl/Cmd keeps the spiral's
inner end at the distance from the center it had when the key went
down, and the outer end follows the pointer. With Alt the number of
segments changes to the nearest whole number that fits, within 2 to
1000, so dragging outward adds segments and dragging inward removes
them. With Ctrl/Cmd the decay changes to fit, within 5 to 99.99
percent, the decays a spiral is drawn with. When both keys are held,
Ctrl/Cmd changes the decay. Up/Down with a key still held move the
held radius to the new segment count. The new values stay when the
key is let go, and as the Spiral tool's segments and decay options
for the next spiral. Both keys also work with a 5 percent decay and
hundreds of segments.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/tests/draw2.rs

@@ -1275,6 +1275,38 @@ fn shape_arc_closed_along_y_closes_through_the_top_left_corner() {
     assert!((area - std::f64::consts::FRAC_PI_4).abs() < 0.01, "{area}");
 }
 
+/// A spiral dragged out with Alt pressed part way gets more segments as its outer end moves out,
+/// and one dragged with Ctrl/Cmd pressed part way gets the decay that keeps its inner end's
+/// distance from the center; the Spiral tool keeps both for the next spiral.
+#[test]
+fn spiral_drags_with_alt_and_ctrl_change_its_segments_and_decay() {
+    let mut s = session();
+    let v = view();
+    let spiral = |s: &mut Session, m: Mods, to: (f64, f64)| {
+        s.pointer(&PointerEvent::new(PointerKind::Down, 100.0, 100.0), v).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Drag, 200.0, 100.0), v).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Drag, 200.0, 100.0).with_mods(m), v).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Drag, to.0, to.1).with_mods(m), v).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Up, to.0, to.1).with_mods(m), v).unwrap();
+        let sp = paths(s).last().unwrap().1.subpaths[0].clone();
+        (sp.anchors.len(), sp.anchors.last().unwrap().p)
+    };
+    s.select_tool("spiral", v).unwrap();
+    let undo = undo_steps(&s);
+    // Alt from a radius of 100 to 200: 10 segments become 13, the outer end at the pointer.
+    let (anchors, outer) = spiral(&mut s, Mods { alt: true, ..Mods::default() }, (300.0, 100.0));
+    assert_eq!(anchors, 14);
+    assert!(near(outer, Point::new(300.0, 100.0)), "{outer:?}");
+    assert_eq!(s.tool_options()["segments"], json!(13));
+    // Ctrl/Cmd from 100 to 50: the decay rises from 80 % to 80 × 2^(1/13) %.
+    let (anchors, outer) = spiral(&mut s, Mods { cmd: true, ..Mods::default() }, (150.0, 100.0));
+    assert_eq!(anchors, 14);
+    assert!(near(outer, Point::new(150.0, 100.0)), "{outer:?}");
+    let decay = s.tool_options()["decay"].as_f64().unwrap();
+    assert!((decay - 80.0 * 2f64.powf(1.0 / 13.0)).abs() < 1e-9, "{decay}");
+    assert_eq!(undo_steps(&s) - undo, 2, "one undo step a spiral");
+}
+
 /// The children of the grid a `shape.rectangularGrid` / `shape.polarGrid` call made, bottom first.
 fn grid(s: &mut Session, cmd: &str, p: serde_json::Value) -> Vec<std::sync::Arc<vectorcraft_doc::Node>> {
     let id = NodeId(s.execute(cmd, &p).unwrap()["id"].as_u64().unwrap());