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>

crates/ocs_gcs/src/lib.rs

@@ -0,0 +1,15 @@
+//! 2D geometric constraint solver for OpenCADStudio sketches.
+//!
+//! A Rust port of FreeCAD's planegcs (LGPL-2.1-or-later), staged bottom-up:
+//! see `/Users/felix/.claude/plans/snoopy-finding-matsumoto.md` for the full
+//! port plan and stage order.
+
+pub mod constraints;
+pub mod diagnosis;
+pub mod geo;
+pub mod graph;
+pub mod qp_eq;
+pub mod solvers;
+pub mod subsystem;
+pub mod system;
+pub mod util;