Anchor Point tool: Shift-drag on an anchor moves it, handles and all

With the Anchor Point tool, a drag from an anchor pulls out smooth handles.
Shift held at the press now moves the anchor instead, its handles keeping
their offsets; Shift kept down during the drag keeps the move at 45° steps
(the constrain angle), released it frees it. A Shift-click that doesn't
move leaves the anchor as it was. A selected blend's spine points follow
the same rule (object.blend.spine.moveAnchor).

The move goes through a new engine command, path.moveAnchor
{id, subpath?, anchor, x, y}, since path.moveAnchors only acts on the
direct-selected anchors and the Anchor Point tool keeps no anchor selection.

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

crates/engine/src/cmd/path.rs

@@ -42,6 +42,15 @@ pub fn specs() -> Vec<CommandSpec> {
             has_doc,
             set_handle
         ),
+        cmd!(
+            "path.moveAnchor",
+            "Move Anchor Point",
+            [],
+            None,
+            "{id, subpath?, anchor, x, y} move one anchor to (x, y), its handles with it",
+            has_doc,
+            move_anchor
+        ),
         cmd!(
             "path.setAnchors",
             "Set Anchors",
@@ -230,6 +239,22 @@ fn set_handle(s: &mut Session, p: &Value) -> Result<Value> {
     ok()
 }
 
+/// `path.moveAnchor`: one anchor to (x, y), its handles keeping their offsets (the Anchor Point
+/// tool's Shift-drag).
+fn move_anchor(s: &mut Session, p: &Value) -> Result<Value> {
+    let id = id_param(p, "id").ok_or_else(|| bad("path.moveAnchor", "missing id"))?;
+    let si = usize_opt(p, "subpath", "path.moveAnchor")?.unwrap_or(0);
+    let ai = p.get("anchor").and_then(Value::as_u64).ok_or_else(|| bad("path.moveAnchor", "missing anchor"))? as usize;
+    let pos = Point::new(f64_req(p, "x", "path.moveAnchor")?, f64_req(p, "y", "path.moveAnchor")?);
+    s.edit("Move", |d, _| {
+        let path = path_mut(d, id)?;
+        let a = path.anchor_mut(si, ai).ok_or_else(|| EngineError::Other("no such anchor".into()))?;
+        *a = a.transform(vectorcraft_geom::Affine::translate(pos - a.p));
+        Ok(())
+    })?;
+    ok()
+}
+
 fn set_anchors(s: &mut Session, p: &Value) -> Result<Value> {
     let id = id_param(p, "id").ok_or_else(|| bad("path.setAnchors", "missing id"))?;
     let subs = p.get("subpaths").and_then(Value::as_array).ok_or_else(|| bad("path.setAnchors", "missing subpaths"))?;