STRETCH: arcs keep their height; splines stretch; real preview

An arc moved only when its centre was in the window, so stretching a
rounded end left the arc where it was or blew it up. Its ends now decide:
both in move it, one in moves that end while the arc keeps its height
(sagitta) over the new chord. Polyline arc segments keep their height the
same way instead of their bulge, spline control and fit points in the
window move, and 3D polylines and points stretch too. Old-style polylines
were tested in X/Z.

The preview moved the tessellated points that happened to be in the
window, which bent arcs out of shape; it now draws the stretched entities
through one shared function the commit also uses, via a new
preview_entities hook, and hides the originals while it shows.

Closes #1851

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

src/command.rs

@@ -2974,6 +2974,13 @@ pub trait CadCommand: Send {
         self.on_preview_wires(pt)
     }
 
+    /// Entities the host draws as the live preview, tessellated like the
+    /// drawing's own (arcs, text, linetypes and all). Commands whose preview
+    /// changes an entity's shape return edited copies of it here.
+    fn preview_entities(&mut self, _document: &codec::CadDocument, _pt: DVec3) -> Vec<EntityType> {
+        Vec::new()
+    }
+
     /// Source entities replaced by the current live preview. The host removes
     /// these from the resident render until the command commits or cancels.
     /// Commands such as COPY keep their sources visible and use the default.