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 <[email protected]>

src/command.rs

@@ -1317,6 +1317,35 @@ pub enum CmdResult {
     ReplaceMany(Vec<(Handle, Vec<EntityType>)>, Vec<EntityType>),
     /// Replace several entities as one undo step while keeping the command active.
     ReplaceManyContinue(Vec<(Handle, Vec<EntityType>)>),
+    /// Add a persistent parametric constraint (`docs/parametric_system_design.md`
+    /// §6.1) to the current scope's `SketchConstraintSet` and trigger its
+    /// first solve; end the command. Unlike `ReplaceMany`/`CommitEntity`,
+    /// nothing here is geometry to add or replace directly — the host adds
+    /// the constraint record, then re-solves through `Scene::bump_entities`'s
+    /// existing chain (`refresh_sketch_constraints`) the same way any later
+    /// edit to these entities will.
+    AddSketchConstraint {
+        kind: crate::scene::sketch_constraints::ConstraintKind,
+        refs: Vec<crate::scene::sketch_constraints::SketchRef>,
+        /// The typed target for a dimensional kind (Distance/Angle/Radius);
+        /// `None` for a purely geometric one.
+        driving_param: Option<f64>,
+        /// Undo-history label, e.g. `"Horizontal constraint"`.
+        label: &'static str,
+    },
+    /// Add a persistent Coincident constraint between whatever sub-entity
+    /// point resolves nearest each picked point (design doc §6.1/§6.2's
+    /// manual Coincident UI — `CoincidentConstraintCommand`,
+    /// `src/modules/draw/constrain/coincident.rs`). Like
+    /// `ReassociateCenterMark`, a `CadCommand` has no document access to
+    /// resolve a marker itself, so it hands the two raw points back for the
+    /// host to resolve (`sketch_constraints::nearest_sketch_point`) and, if
+    /// both land near a real point, build an `AddSketchConstraint` from.
+    AddCoincidentConstraint {
+        point_a: DVec3,
+        point_b: DVec3,
+        label: &'static str,
+    },
     /// Attach one smart centre mark to a newly selected circular source.
     ReassociateCenterMark {
         target: Handle,