Add 8 geometric constraint kinds and an opt-in DWG-native persistence toggle

New ConstraintKind variants — Concentric, CenterPoint, Colinear, Midpoint,
Fixed, PointOnCurve, EqualDistance, Symmetric — each built entirely from
existing ocs_gcs (FreeCAD planegcs) primitives, no new solver math needed.
build_constraint now returns Vec<Rc<dyn Constraint>> instead of
Option<Rc<...>>, generalizing the old Coincident-only "needs a second
system-level constraint" special case to every kind that needs one.
Wired into the ribbon (7 new buttons), a new point+pre-selected-entity
command for CenterPoint/Midpoint/PointOnCurve, a new 4-point command for
EqualDistance, and the existing DWG-native class-name mapping (all 8 reuse
the same generic "plain Geometrical node" path already built for
Coincident/Horizontal/etc.). Deliberately not modeled: Normal and
EqualCurvature, which for this app's Line/Circle-only entity model would
just be Perpendicular and Equal wearing different DWG class names.

Also makes DWG-native constraint persistence opt-in (Options > General,
off by default) rather than unconditional: a scope that doesn't yet have
the native graph is only given one when the setting is on, but a scope
that already has one — from an earlier save, or a file that came from
real AutoCAD — keeps getting it resynced regardless of the setting, so
toggling it off never freezes a stale graph in place.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/modules/draw/mod.rs

@@ -34,7 +34,11 @@ impl CadModule for DrawModule {
         };
         use crate::modules::insert::{create_block, insert_block};
         use clipboard::{copy_clip, cut, paste};
-        use constrain::{angle_tool, coincident_tool, distance_tool, equal, horizontal, parallel, perpendicular, tangent, vertical};
+        use constrain::{
+            angle_tool, center_point_tool, coincident_tool, colinear, concentric, distance_tool, equal,
+            equal_distance_tool, fixed, horizontal, midpoint_tool, parallel, perpendicular, point_on_curve_tool,
+            symmetric, tangent, vertical,
+        };
         use draw::{arc, circle, ellipse, hatch, line, polyline, shapes};
         use groups::{group, ungroup};
         use layers::{
@@ -134,6 +138,14 @@ impl CadModule for DrawModule {
                         coincident_tool::tool().into(),
                         distance_tool::tool().into(),
                         angle_tool::tool().into(),
+                        concentric::tool().into(),
+                        colinear::tool().into(),
+                        fixed::tool().into(),
+                        symmetric::tool().into(),
+                        center_point_tool::tool().into(),
+                        midpoint_tool::tool().into(),
+                        point_on_curve_tool::tool().into(),
+                        equal_distance_tool::tool().into(),
                     ],
                 },
                 RibbonGroup {