Persist map selections as derived datasets

docs/API_CONTRACTS.md

@@ -406,6 +406,40 @@ Rules:
 - Results are generated from persisted PostGIS `vector_features`, not from client-side map data.
 - The response is capped by `limit` and returns `truncated=true` when more matching rows exist.
 
+### POST `/api/v1/projects/{project_id}/datasets/{dataset_id}/vector/select/derive`
+
+Persists a bbox selection as a new derived vector dataset and indexes the
+selected output into `vector_features`.
+
+Request:
+
+```json
+{
+  "bbox": {
+    "min_x": 5.0,
+    "min_y": 51.0,
+    "max_x": 5.1,
+    "max_y": 51.1,
+    "crs": "EPSG:4326"
+  },
+  "limit": 250,
+  "output_name": "selected-buildings"
+}
+```
+
+Response: `DatasetRead` in the canonical API envelope.
+
+Rules:
+
+- Only vector/GeoJSON datasets are supported.
+- Coordinates are EPSG:4326 longitude/latitude.
+- The new dataset uses `dataset_role="derived"`, `source="operation:selection"`,
+  `source_name="map_selection"` and `derived_from_dataset_id` pointing to the
+  source dataset.
+- The persisted GeoJSON properties retain source provenance as
+  `source_dataset_id` and `source_vector_feature_id`.
+- Empty selections return `VECTOR_OPERATION_EMPTY_RESULT` and do not create a dataset.
+
 ### GET `/api/v1/projects/{project_id}/datasets/{dataset_id}/content`
 
 Returns stored vector dataset content through the canonical API envelope.