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/pick/grip.rs

@@ -217,8 +217,10 @@ pub fn find_hit_grip(
 /// Project an f64 world point with an explicit relative-to-eye `(view_rot,
 /// eye)` pair — the camera-less form of `Camera::project`. Used by the
 /// in-viewport editing path, which supplies a *composed* model→screen view
-/// (see `Scene::composed_viewport_view`) instead of a real camera.
-fn project_rte(world: DVec3, view_rot: Mat4, eye: DVec3, bounds: Rectangle) -> Option<Vec2> {
+/// (see `Scene::composed_viewport_view`) instead of a real camera. `pub(crate)`
+/// so any other single-point world→screen projection (e.g. constraint-glyph
+/// anchors) can reuse it instead of re-deriving the same NDC math.
+pub(crate) fn project_rte(world: DVec3, view_rot: Mat4, eye: DVec3, bounds: Rectangle) -> Option<Vec2> {
     let rel = (world - eye).as_vec3();
     let clip = view_rot * rel.extend(1.0);
     if clip.w.abs() < 1e-9 {