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/modules/draw/constrain/coincident.rs

@@ -0,0 +1,73 @@
+//! CCONSTRAINT — the manual Coincident UI (design doc §6.1's open item):
+//! unlike the rest of this module's plain select-and-click constraints,
+//! Coincident addresses a *point* on an entity (an endpoint, a circle/arc
+//! center), not the whole entity, so it needs two point picks instead of a
+//! selection.
+//!
+//! A `CadCommand` has no document access (see `value.rs`'s module doc
+//! comment for the same constraint on `DCONSTRAINT`/`ACONSTRAINT`), so this
+//! command only accumulates the two raw points `on_point` receives and hands
+//! them back as `CmdResult::AddCoincidentConstraint` — the host resolves
+//! each point to a real `SketchRef` (`sketch_constraints::nearest_sketch_point`)
+//! and adds the constraint, the same split `ReassociateCenterMark` already
+//! uses for a similar reason.
+
+use glam::DVec3;
+
+use crate::command::{CadCommand, CmdResult};
+use crate::modules::{IconKind, ModuleEvent, ToolDef};
+
+pub mod coincident_tool {
+    use super::*;
+    pub fn tool() -> ToolDef {
+        ToolDef {
+            id: "CCONSTRAINT",
+            label: "Coincident",
+            icon: IconKind::Glyph("≡"),
+            event: ModuleEvent::Command("CCONSTRAINT".to_string()),
+        }
+    }
+}
+
+#[derive(Default)]
+pub struct CoincidentConstraintCommand {
+    first: Option<DVec3>,
+}
+
+impl CoincidentConstraintCommand {
+    pub fn new() -> Self {
+        Self::default()
+    }
+}
+
+impl CadCommand for CoincidentConstraintCommand {
+    fn name(&self) -> &'static str {
+        "CCONSTRAINT"
+    }
+
+    fn prompt(&self) -> String {
+        if self.first.is_none() {
+            "COINCIDENT  Specify first point:".to_string()
+        } else {
+            "COINCIDENT  Specify second point:".to_string()
+        }
+    }
+
+    fn on_point(&mut self, pt: DVec3) -> CmdResult {
+        match self.first {
+            None => {
+                self.first = Some(pt);
+                CmdResult::NeedPoint
+            }
+            Some(first) => CmdResult::AddCoincidentConstraint { point_a: first, point_b: pt, label: "Coincident constraint" },
+        }
+    }
+
+    fn on_enter(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+
+    fn on_escape(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+}