fix(map): make area analysis scale-aware

docs/API_CONTRACTS.md

@@ -881,6 +881,24 @@ Rules:
 - `limit` is bounded to `1..1000`. Municipality-scale clients must page spatially by viewport instead of requesting an unbounded municipality FeatureCollection.
 - The Map workspace uses this existing endpoint for vector datasets above 5,000 features. It starts delivery at zoom level 14, debounces `moveend` requests and explicitly reports `truncated=true` as a request to zoom further in. This is a client delivery policy, not a second API or persistence path.
 
+### POST `/api/v1/projects/{project_id}/datasets/vector/partitions/select`
+
+Combines `1..16` bounded vector acquisitions of one governed source product
+into one read-only selection result. The request uses the same `bbox`, optional
+`area_id` and `limit` contract as vector selection plus `dataset_ids`.
+
+The backend rejects mixed projects, non-ready vector datasets and partitions
+whose `source_name` or `product_key` differs. PostGIS calculates area and length
+metrics across all persisted tile geometries. `total_feature_count` is
+deduplicated by provider `source_feature_id` where available so a source object
+crossing a tile edge is not presented as two objects. The response includes
+`partition_count`, `source_name` and the exact `dataset_ids` used.
+
+The Map workbench uses this route only for regional selections up to 50 by 50
+kilometres. Provider calls remain individually bounded below 20 kilometres;
+larger overview selections do not fan out into unbounded high-resolution
+downloads.
+
 ### 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