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/command.rs

@@ -1592,6 +1592,14 @@ pub enum CmdResult {
         direction: acadrust::types::Vector3,
         label: &'static str,
     },
+    /// Adds an Equal relation from `first` to each of `others` (the second
+    /// object, or a Multiple set). The host resizes every follower to the
+    /// first object's length or radius before the relation holds it there.
+    AddEqualConstraint {
+        first: crate::scene::parametric_constraints::ParametricRef,
+        others: Vec<crate::scene::parametric_constraints::ParametricRef>,
+        label: &'static str,
+    },
     /// Adds a point or object symmetry relation around a picked line. The
     /// first reference and axis remain fixed during initial placement.
     AddSymmetricConstraint {
@@ -2894,6 +2902,7 @@ mod constraint_registry_tests {
             "QCONSTRAINT",
             "GCPERPENDICULAR",
             "ECONSTRAINT",
+            "GCEQUAL",
             "TCONSTRAINT",
             "GCCONCENTRIC",
             "NRCONSTRAINT",