Bring the Equal constraint to reference parity

Equal was applied through a generic gather-then-apply flow and, with the
default size-retaining solve mode, never changed anything: the solver
held both sizes and the relation could not be met. Now:

- GCEQUAL joins ECONSTRAINT and GEOMCONSTRAINT Equal; both objects are
  picked inside the command (`Select first object or [Multiple]:`,
  `Select second object:`; Multiple gathers `Select objects to make equal
  to the first:`), a selection made beforehand is not used, a miss says
  `No object found.` and re-asks, an unsupported object or the same object
  twice is reported and re-asked, a repeat is refused.
- The follower is resized in place before the relation is added: a line or
  straight polyline segment keeps its start and direction and its end
  moves to the first object's length; a circle or arc keeps its center and
  takes the radius.
- During edits the side that is not being edited follows the same rule:
  its size is not retained, its start (or center) and other vertices are
  held and its direction kept, so only its length or radius changes; a
  chain of Equal relations follows along.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/app/commands/draw.rs

@@ -1569,8 +1569,17 @@ impl OpenCADStudio {
                 }
             }
 
-            "PCONSTRAINT" | "ECONSTRAINT" | "LCONSTRAINT"
-            | "NRCONSTRAINT" => {
+            "ECONSTRAINT" | "GCEQUAL" => {
+                use crate::modules::parametric::EqualConstraintCommand;
+                // Both objects are picked inside the command, as in the
+                // reference; a selection made beforehand is not used.
+                self.tabs[i].scene.deselect_all();
+                let command = EqualConstraintCommand::new();
+                self.command_line.push_info(&command.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(command));
+            }
+
+            "PCONSTRAINT" | "LCONSTRAINT" | "NRCONSTRAINT" => {
                 let handles = self.tabs[i].scene.selected_handles_in_order();
                 if handles.is_empty() {
                     use crate::modules::draw::select::SelectObjectsCommand;
@@ -1587,8 +1596,7 @@ impl OpenCADStudio {
                     let (kind, label) = match cmd {
                         "PCONSTRAINT" => (ConstraintKind::Parallel, "Parallel constraint"),
                         "LCONSTRAINT" => (ConstraintKind::Colinear, "Colinear constraint"),
-                        "NRCONSTRAINT" => (ConstraintKind::Normal, "Normal constraint"),
-                        _ => (ConstraintKind::Equal, "Equal constraint"),
+                        _ => (ConstraintKind::Normal, "Normal constraint"),
                     };
                     return Some(self.apply_cmd_result(CmdResult::AddParametricConstraint {
                         kind,