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/app/config.rs

@@ -435,10 +435,25 @@ pub struct StartConfig {
     pub section: super::StartSection,
 }
 
-#[derive(Clone, PartialEq, Serialize, Deserialize, Default)]
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
 #[serde(default)]
 pub struct RibbonConfig {
     pub collapse: CollapseMode,
+    /// Font size (px) of the caption under a large ribbon button's icon.
+    pub label_font_size: i32,
+    /// Font size (px) of a panel's own title ("Draw", "Modify", "Layers", …)
+    /// shown beneath its tools.
+    pub group_title_font_size: i32,
+}
+
+impl Default for RibbonConfig {
+    fn default() -> Self {
+        Self {
+            collapse: CollapseMode::default(),
+            label_font_size: 10,
+            group_title_font_size: 9,
+        }
+    }
 }
 
 impl AppConfig {