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/dimension_assoc.rs

@@ -264,7 +264,13 @@ fn polyline_arc_center(entity: &EntityType, segment: usize) -> Option<Vector3> {
     }
 }
 
-fn source_points(entity: &EntityType) -> Vec<Vector3> {
+/// Ordered, named points for an entity (line start/end, polyline vertices,
+/// ...), indexed by the same non-negative GsMarker convention
+/// `AssocDimensionReference::main_gs_marker` uses. Promoted to
+/// `pub(crate)` so `sketch::constraint_set` can address constraint
+/// endpoints the same way associative dimensions already address theirs,
+/// rather than inventing a second sub-element scheme.
+pub(crate) fn source_points(entity: &EntityType) -> Vec<Vector3> {
     match entity {
         EntityType::Line(line) => vec![line.start, line.end],
         EntityType::Arc(arc) => vec![arc.start_point_wcs(), arc.end_point_wcs()],