Add persistent sketch-constraint system (ocs_gcs port) and named-parameters handoff docs

Ports FreeCAD's planegcs 2D geometric constraint solver to Rust as a new
crates/ocs_gcs workspace member, and wires it into the document/scene model
as a persistent SketchConstraintSet: Coincident, Horizontal/Vertical,
Parallel/Perpendicular, Equal, Tangent, Distance, Angle and Radius
constraints that re-solve live (including mid-drag, not just on release),
render glyphs and a DOF badge, flag conflicts with a one-click resolver
pill, and round-trip through save/load via an XRecord blob.

Also includes docs/command_reference.md (a statically generated reference
of every registered command and its prompt sequence) and
docs/named_parameters_design.md + docs/backlog.md, which hand off the next
queued feature (a named-parameter/expression table) and consolidate every
deliberate scope-down and open gap found along the way, so a fresh session
has a complete starting point.

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

src/scene/modify.rs

@@ -752,6 +752,11 @@ impl Scene {
             .map(|&h| (h, ChangeKind::Added))
             .collect();
         self.bump_entities(&changes);
+        // Design doc §12: a constraint entirely between duplicated entities
+        // follows them (Coincident between two copied points, Parallel
+        // between two copied lines, ...); one straddling a duplicated and a
+        // non-duplicated entity can't sensibly follow, so is left alone.
+        self.duplicate_sketch_constraints_for(&handle_map);
         self.refresh_meshes_for_handles(&refresh_solid_handles);
         new_handles
     }