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/app/automation.rs

@@ -268,6 +268,7 @@ impl OpenCADStudio {
             "new" => {
                 let i = self.active_tab;
                 self.tabs[i].scene.document = acadrust::CadDocument::new();
+                self.tabs[i].scene.sketch_constraints.clear();
                 self.tabs[i].scene.deselect_all();
                 self.tabs[i].current_path = None;
                 // The headless session starts on the welcome (Start) tab, which
@@ -293,6 +294,7 @@ impl OpenCADStudio {
                     Ok(doc) => {
                         let i = self.active_tab;
                         self.tabs[i].scene.document = doc;
+                        self.tabs[i].scene.load_sketch_constraints_from_document();
                         self.tabs[i].scene.deselect_all();
                         crate::app::style_ops::ensure_standard_styles(
                             &mut self.tabs[i].scene.document,