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>

Cargo.toml

@@ -11,12 +11,14 @@ members = [
     "crates/dwg-thumbnailer",
     "crates/dwg-thumbnailer-win",
     "crates/ocs_doc_api",
+    "crates/ocs_gcs",
 ]
 
 [target.'cfg(windows)'.build-dependencies]
 winresource = "0.1"
 
 [dependencies]
+ocs_gcs = { path = "crates/ocs_gcs" }
 serde_json = "1"
 serde = { version = "1", features = ["derive"] }
 base64 = "0.22"