solid.mirror: mirror bodies of placed components where they are shown (#1896)

Thanks @NOWN-DEV.

crates/engine/src/frames.rs

@@ -36,7 +36,8 @@ enum K {
     /// component frame, whichever component the geometry under it belongs to (a sketch in the
     /// root on a moved component's face finds the face where it is shown).
     Here,
-    /// A plane object for a new sketch or canvas: into the active component's frame.
+    /// A plane object for a new sketch or canvas (or a mirror's copies): into the active
+    /// component's frame.
     HerePlane,
 }
 
@@ -59,7 +60,10 @@ fn table(id: &str) -> &'static [(&'static str, K)] {
         "solid.press_pull" => &[("face", Pick), ("edges", Picks)],
         "solid.replace_face" => &[("faces", Picks), ("target_face", Pick), ("target", Plane), ("body", Body)],
         "solid.align" => &[("from", Pick), ("from_face", Pick), ("to", Pick), ("to_face", Pick), ("bodies", Body)],
-        "solid.mirror" | "solid.split_body" => &[("plane", Plane), ("bodies", Body), ("body", Body)],
+        // A mirror keeps the placement its bodies were picked at (`cmd::features::placed_bodies`);
+        // its plane is in the active component, where the copies go.
+        "solid.mirror" => &[("plane", HerePlane)],
+        "solid.split_body" => &[("plane", Plane), ("bodies", Body), ("body", Body)],
         "plastic.boss" | "plastic.snap_fit" | "plastic.rest" => {
             &[("position", Pick), ("direction", Dir), ("hook", Dir), ("along", Dir), ("body", Body)]
         }