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/modules/draw/mod.rs

@@ -2,6 +2,7 @@
 
 mod changelog;
 pub mod clipboard;
+pub mod constrain;
 pub mod defaults;
 mod donate;
 pub mod draw;
@@ -33,6 +34,7 @@ impl CadModule for DrawModule {
         };
         use crate::modules::insert::{create_block, insert_block};
         use clipboard::{copy_clip, cut, paste};
+        use constrain::{angle_tool, coincident_tool, distance_tool, equal, horizontal, parallel, perpendicular, tangent, vertical};
         use draw::{arc, circle, ellipse, hatch, line, polyline, shapes};
         use groups::{group, ungroup};
         use layers::{
@@ -120,6 +122,20 @@ impl CadModule for DrawModule {
                         offset::tool().into(),
                     ],
                 },
+                RibbonGroup {
+                    title: "Constraints",
+                    tools: vec![
+                        horizontal::tool().into(),
+                        vertical::tool().into(),
+                        parallel::tool().into(),
+                        perpendicular::tool().into(),
+                        equal::tool().into(),
+                        tangent::tool().into(),
+                        coincident_tool::tool().into(),
+                        distance_tool::tool().into(),
+                        angle_tool::tool().into(),
+                    ],
+                },
                 RibbonGroup {
                     title: "Annotation",
                     tools: vec![