ocs_doc_api: outstanding methods — loft + bulge-arc polyline profiles

- Loft{profiles} op (append-only); executor resolves each profile to curve sets
  all-or-nothing; backend loft() -> brep::loft on XY plane; facade solids().loft.
- entity_to_profile_curves: convert LwVertex.bulge to geom2d::Curve::Arc via
  bulge_arc_segment (theta = 4*atan(bulge)), so bulged polyline profiles sweep
  correctly instead of erroring Unsupported.
- spec loft; api_reference + binding_schema regenerated; mocks + baseline.
- Tests: loft_two_profiles_produces_solid, bulge_arc_polyline_profile_converts_to_arc
  (semicircle radius=5 for bulge=1) -> 612 host lib + 3 consistency green.

crates/ocs_doc_api/src/backend.rs

@@ -84,6 +84,10 @@ pub trait DocApiBackend {
     /// Add a `RASTER_IMAGE` (host auto-registers the ImageDefinition).
     fn add_raster_image(&mut self, spec: &crate::ops::RasterImageSpec) -> ApiResult<ObjectId>;
 
+    /// Loft a solid through >= 2 profile curve sets (each already resolved to
+    /// `geom2d::Curve`s on the XY plane). Returns the fresh `ObjectId`.
+    fn loft(&mut self, sections: &[Vec<cadkernel::geom2d::Curve>]) -> ApiResult<ObjectId>;
+
     /// Can `id` be modified in place right now (exists, is the expected family,
     /// not on a locked layer)? Read-only pre-check used before mutations.
     /// Default: existence + not-locked (backends narrow the family check).