ocs_doc_api: Phase 2b-b (Hatch)

- CreateHatch(HatchSpec) op + GetHatchBoundary query (append-only).
- HatchSpec{boundary: Vec<[f64;2]>, solid}; executor validates >= 3 boundary points.
- backend add_hatch (Line-edge BoundaryPath, solid/pattern) + hatch_boundary
  (reconstruct loops from Line edges; non-line edges -> Unsupported).
- Hatch bounds from boundary edges; facade create_hatch + Entity handle.
- spec Hatch family; api_reference + binding_schema regenerated; mocks + baseline.
- Test phase2b_hatch_create_boundary_bounds_delete -> 606 host lib + 8 crate + 3 consistency green.

crates/ocs_doc_api/src/backend.rs

@@ -54,6 +54,12 @@ pub trait DocApiBackend {
     /// The text content of a Text/MText annotation. `Unsupported` for other kinds.
     fn text_content(&self, id: ObjectId) -> ApiResult<String>;
 
+    /// Add a `HATCH` (single closed polyline boundary); returns the fresh `ObjectId`.
+    fn add_hatch(&mut self, spec: &crate::ops::HatchSpec) -> ApiResult<ObjectId>;
+
+    /// A hatch's boundary loops (outer + islands) as closed polylines.
+    fn hatch_boundary(&self, id: ObjectId) -> ApiResult<Vec<Vec<[f64; 2]>>>;
+
     /// 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).