ocs_doc_api: Phase 2b-c (linear Dimension) — Phase 2 complete

- CreateDimensionLinear(DimensionSpec) op + GetDimensionMeasurement query (append-only).
- backend add_dimension_linear (Dimension::Linear via DimensionLinear::new) +
  dimension_measurement (linear/radius/diameter distance).
- Dimension bounds from measured + definition points; Dimension handle + measurement().
- spec Dimension family; api_reference + binding_schema regenerated; mocks + baseline.
- Test phase2c_dimension_linear_create_measurement_bounds -> 607 host lib + 8 crate
  + 3 consistency green. Phase 2 (full 2D + annotations) is now complete.

crates/ocs_doc_api/src/backend.rs

@@ -60,6 +60,12 @@ pub trait DocApiBackend {
     /// A hatch's boundary loops (outer + islands) as closed polylines.
     fn hatch_boundary(&self, id: ObjectId) -> ApiResult<Vec<Vec<[f64; 2]>>>;
 
+    /// Add a linear `DIMENSION`; returns the fresh `ObjectId`.
+    fn add_dimension_linear(&mut self, spec: &crate::ops::DimensionSpec) -> ApiResult<ObjectId>;
+
+    /// A dimension's measured value (distance for linear/radius, degrees for angular).
+    fn dimension_measurement(&self, id: ObjectId) -> ApiResult<f64>;
+
     /// 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).