Match Fix geometric constraint behavior

1) FXCONSTRAINT (and the new GCFIX name) prompts "Select point or [Object]
   <Object>:" — a pick fixes that constraint point; Enter or O asks for one
   line, polyline segment, circle, arc, ellipse or spline; misses and
   unsupported objects report the reference messages and re-prompt.
2) Point, block, text, MText, attribute and table insertion points are
   addressable constraint points.
3) Re-applying Fixed to the same point or object reports that the constraint
   already exists instead of adding a duplicate.
4) A persistent Fixed holds its pre-edit geometry through MOVE, ROTATE,
   SCALE, STRETCH, grip and Properties edits: fixed points stay, free points
   of a partly fixed entity follow the edit, and size retention skips fixed
   entities.
5) Object mode on a polyline fixes the picked segment (natively persisted)
   instead of the whole polyline; arc segments pin endpoints, center and
   radius.
6) Ribbon label "Fix" with a padlock icon.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/modules/parametric/tools.rs

@@ -136,7 +136,7 @@ pub mod fixed {
     pub fn tool() -> ToolDef {
         ToolDef {
             id: "FXCONSTRAINT",
-            label: "Fixed",
+            label: "Fix",
             icon: IconKind::Svg(include_bytes!("../../../assets/icons/constrain/fixed.svg")),
             event: ModuleEvent::Command("FXCONSTRAINT".to_string()),
         }
@@ -171,6 +171,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "NRCONSTRAINT",
         "LCONSTRAINT",
         "FXCONSTRAINT",
+        "GCFIX",
         "SYCONSTRAINT",
     ]
 });