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/ui/window/options.rs

@@ -31,6 +31,7 @@ impl<T> fmt::Display for Labelled<T> {
 pub fn view_window<'a>(
     default_save_format: &'a str,
     file_assoc_enabled: bool,
+    write_dwg_native_constraints: bool,
     ui_theme: &'a UiThemeConfig,
     theme_color_inputs: &'a [String; 6],
     language: crate::i18n::Language,
@@ -190,6 +191,24 @@ pub fn view_window<'a>(
         ))
         .size(11)
         .width(sizing.width),
+        Space::new().height(14),
+        row![
+            iced::widget::checkbox(write_dwg_native_constraints)
+                .on_toggle(Message::WriteDwgNativeConstraintsChanged)
+                .size(15),
+            text(crate::t!("Write AutoCAD-compatible constraint objects on save")).size(12),
+        ]
+        .spacing(8)
+        .align_y(iced::Center),
+        Space::new().height(6),
+        text(crate::t!(
+            "Saves sketch constraints as AutoCAD/BricsCAD's own native objects too, alongside \
+             this app's own format, so other CAD software recognizes them. Off by default — \
+             adds some file size on every save. A drawing that already has these objects keeps \
+             them up to date regardless of this setting."
+        ))
+        .size(11)
+        .width(sizing.width),
     ]
     .spacing(0)
     .width(sizing.width);