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/app/settings.rs

@@ -225,6 +225,15 @@ pub struct UserSettings {
     /// When true (default), the app (re)registers itself as a .dwg/.dxf/.bak
     /// handler on every launch. Toggle with the FILEASSOC command.
     pub file_assoc_enabled: bool,
+    /// When true, saving a drawing also writes its sketch constraints as
+    /// AutoCAD's own native object graph (`scene::dwg_native_constraints`),
+    /// not just the app's own XRecord format. Off by default — opt-in, since
+    /// it adds file weight every save whether or not AutoCAD/BricsCAD
+    /// interop is needed. See `write_dwg_native_constraints` on the app
+    /// struct for the full contract (a drawing that already has this native
+    /// graph keeps it synced regardless of this setting).
+    #[serde(default)]
+    pub write_dwg_native_constraints: bool,
     /// Minutes between autosaves to a `.sv$` recovery file (SAVETIME command).
     /// 0 disables autosave.
     pub savetime_min: i32,
@@ -338,6 +347,7 @@ impl Default for UserSettings {
             textfill: true,
             backup_on_save: true,
             file_assoc_enabled: true,
+            write_dwg_native_constraints: false,
             savetime_min: 10,
             default_save_format: crate::io::DEFAULT_SAVE_FORMAT.to_string(),
             pick_add: true,