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/constrain/equal_distance.rs

@@ -0,0 +1,67 @@
+//! EDCONSTRAINT — AutoCAD's fourth `Equal` sub-kind
+//! (`ACEQUALDISTANCECONSTRAINT`): the distance between one point pair
+//! equals the distance between another, rather than two whole entities
+//! being equal length/radius. Needs four point picks (unlike the two
+//! whole-entity selections `ECONSTRAINT` uses), so — like
+//! `CoincidentConstraintCommand` — it accumulates raw points and hands them
+//! back for the host to resolve (`CadCommand` has no document access).
+
+use glam::DVec3;
+
+use crate::command::{CadCommand, CmdResult};
+use crate::modules::{IconKind, ModuleEvent, ToolDef};
+
+pub mod equal_distance_tool {
+    use super::*;
+    pub fn tool() -> ToolDef {
+        ToolDef {
+            id: "EDCONSTRAINT",
+            label: "Equal Distance",
+            icon: IconKind::Glyph("≐"),
+            event: ModuleEvent::Command("EDCONSTRAINT".to_string()),
+        }
+    }
+}
+
+#[derive(Default)]
+pub struct EqualDistanceConstraintCommand {
+    points: Vec<DVec3>,
+}
+
+impl EqualDistanceConstraintCommand {
+    pub fn new() -> Self {
+        Self::default()
+    }
+}
+
+impl CadCommand for EqualDistanceConstraintCommand {
+    fn name(&self) -> &'static str {
+        "EDCONSTRAINT"
+    }
+
+    fn prompt(&self) -> String {
+        match self.points.len() {
+            0 => "EQUAL DISTANCE  Specify first point of first pair:".to_string(),
+            1 => "EQUAL DISTANCE  Specify second point of first pair:".to_string(),
+            2 => "EQUAL DISTANCE  Specify first point of second pair:".to_string(),
+            _ => "EQUAL DISTANCE  Specify second point of second pair:".to_string(),
+        }
+    }
+
+    fn on_point(&mut self, pt: DVec3) -> CmdResult {
+        self.points.push(pt);
+        if self.points.len() < 4 {
+            return CmdResult::NeedPoint;
+        }
+        let [a, b, c, d] = self.points[..] else { unreachable!("just checked len == 4") };
+        CmdResult::AddEqualDistanceConstraint { points: [a, b, c, d], label: "Equal distance constraint" }
+    }
+
+    fn on_enter(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+
+    fn on_escape(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+}