ocs_doc_api: Phase 2c-iii — 2-line angular dimension (all dimension sub-types done)

- CreateDimensionAngular2Ln(DimensionAngularSpec) op (append-only).
- backend add_dimension_angular2ln (DimensionAngular2Ln::new computes angle).
- facade create_dimension_angular2ln; spec constructor; api_reference + binding_schema
  regenerated; mocks + baseline.
- IMPLEMENTATION.md: fix stale Status line (all 5 phases complete, current test
  counts); mark all dimension sub-types done.
- Test phase2ciii_dimension_angular2ln (90-degree 2-line angle) -> 616 host lib +
  3 consistency green.

crates/ocs_doc_api/src/backend.rs

@@ -78,6 +78,9 @@ pub trait DocApiBackend {
     /// Add a `TABLE` (rows × columns grid); returns the fresh `ObjectId`.
     fn add_table(&mut self, spec: &crate::ops::TableSpec) -> ApiResult<ObjectId>;
 
+    /// Add a 2-line angular `DIMENSION`; returns the fresh `ObjectId`.
+    fn add_dimension_angular2ln(&mut self, spec: &crate::ops::DimensionAngularSpec) -> ApiResult<ObjectId>;
+
     /// A dimension's measured value (distance for linear/radius, degrees for angular).
     fn dimension_measurement(&self, id: ObjectId) -> ApiResult<f64>;