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/spec/entities.toml

@@ -311,6 +311,27 @@ kind = "query"
 query = "GetHatchBoundary"
 returns = "Vec<Vec<[f64; 2]>>"
 
+# ============================================================================
+# DIMENSION FAMILY (phase 2c) — linear dimensions
+# ============================================================================
+[[family]]
+name = "Dimension"
+acadrust_variant = "Dimension"
+handle = "Dimension"
+collection = "curves"
+description = "Linear dimension between two measured points. measurement() reads the distance; bounds from the measured + definition points."
+
+[[family.constructor]]
+name = "create_dimension_linear"
+args = [{ name = "first_point", ty = "[f64; 3]" }, { name = "second_point", ty = "[f64; 3]" }, { name = "definition_point", ty = "[f64; 3]" }]
+op = "CreateDimensionLinear"
+
+[[family.method]]
+name = "measurement"
+kind = "query"
+query = "GetDimensionMeasurement"
+returns = "f64"
+
 # ============================================================================
 # CONTAINER FAMILY (phase 3) — block references
 # ============================================================================