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/command.rs

@@ -1360,6 +1360,32 @@ pub enum CmdResult {
         point_b: DVec3,
         label: &'static str,
     },
+    /// Add a persistent `CenterPoint`/`Midpoint`/`PointOnCurve` constraint
+    /// (`crate::modules::draw::constrain::point_on_entity`) between one
+    /// picked point and `target`, a whole entity selected before the tool
+    /// ran. Like `AddCoincidentConstraint`, the host resolves `point` via
+    /// `sketch_constraints::nearest_sketch_point` (a `CadCommand` has no
+    /// document access) and, for `CenterPoint` specifically, addresses
+    /// `target` via its center marker rather than as a whole entity — see
+    /// `ConstraintKind::CenterPoint`'s own doc comment for why it's the
+    /// same solve as `Coincident`/`Concentric` under a different DWG-native
+    /// class name.
+    AddPointOnEntityConstraint {
+        point: DVec3,
+        target: Handle,
+        kind: crate::scene::sketch_constraints::ConstraintKind,
+        label: &'static str,
+    },
+    /// Add a persistent `EqualDistance` constraint
+    /// (`crate::modules::draw::constrain::equal_distance`): the distance
+    /// between `points[0]`/`points[1]` equals the distance between
+    /// `points[2]`/`points[3]`. The host resolves each point via
+    /// `sketch_constraints::nearest_sketch_point`, same reasoning as
+    /// `AddCoincidentConstraint`.
+    AddEqualDistanceConstraint {
+        points: [DVec3; 4],
+        label: &'static str,
+    },
     /// Attach one smart centre mark to a newly selected circular source.
     ReassociateCenterMark {
         target: Handle,