ocs_doc_api: Phase 2b-a (Text/MText annotations)

- CreateText/CreateMText/SetTextContent ops + GetTextContent query (append-only).
- backend add_text/add_mtext/set_text_content/text_content (Text.value/MText.value).
- facade create_text/create_mtext + Text/MText handles with content()/set_content().
- Text/MText coarse bounds + transform (insertion point); ensure_transformable.
- spec Text/MText families; api_reference + binding_schema regenerated; mocks.
- can_modify backend hook (exists + not-locked) for SetTextContent validation.
- consistency baseline extended with the appended variants.
- Test phase2b_text_mtext_create_content_transform -> 605 host lib + 3 consistency green.

crates/ocs_doc_api/examples/common/mod.rs

@@ -21,6 +21,8 @@ pub struct MockBackend {
     kinds: HashMap<ObjectId, String>,
     /// Stored 2D curve specs so `bounds()` works on non-solid entities too.
     curves: HashMap<ObjectId, Curve2Spec>,
+    /// Stored text content for Text/MText annotations.
+    text_values: HashMap<ObjectId, String>,
 }
 
 impl MockBackend {
@@ -133,6 +135,27 @@ impl DocApiBackend for MockBackend {
     fn add_viewport(&mut self, _spec: &ocs_doc_api::ops::ViewportSpec) -> ApiResult<ObjectId> {
         Ok(self.alloc("Viewport"))
     }
+    fn add_text(&mut self, spec: &ocs_doc_api::ops::TextSpec) -> ApiResult<ObjectId> {
+        let id = self.alloc("Text");
+        self.text_values.insert(id, spec.value.clone());
+        Ok(id)
+    }
+    fn add_mtext(&mut self, spec: &ocs_doc_api::ops::MTextSpec) -> ApiResult<ObjectId> {
+        let id = self.alloc("MText");
+        self.text_values.insert(id, spec.value.clone());
+        Ok(id)
+    }
+    fn set_text_content(&mut self, id: ObjectId, value: &str) -> ApiResult<()> {
+        if let std::collections::hash_map::Entry::Occupied(mut e) = self.text_values.entry(id) {
+            e.insert(value.to_string());
+            Ok(())
+        } else {
+            Err(ApiError::UnknownId(id))
+        }
+    }
+    fn text_content(&self, id: ObjectId) -> ApiResult<String> {
+        self.text_values.get(&id).cloned().ok_or(ApiError::UnknownId(id))
+    }
     fn add_vertex(&mut self, id: ObjectId, _at: usize, _point: [f64; 3]) -> ApiResult<()> {
         if self.entity_exists(id) {
             Ok(())