Add Revit CSV preview API

docs/circuit-list-editor-api.md

@@ -389,3 +389,22 @@ inverse command. The former direct restore/reconnect endpoints are removed.
 
 `displayName` is included in the comparison but is not selected by default in the UI. Updating a
 project device never triggers synchronization implicitly.
+
+## Revit CSV Foundation
+
+- `GET /projects/:projectId/external-csv/configuration`
+  - returns the complete versioned project configuration or `null`
+- `PUT /projects/:projectId/external-csv/configuration`
+  - request: `expectedRevision` and the complete validated configuration
+  - executes `external-csv-configuration.update`
+  - returns configuration, revision and project history state
+- `POST /projects/:projectId/external-csv/preview`
+  - request: file name and Base64-encoded CSV content up to 18 MB
+  - requires a stored project configuration
+  - returns transport metadata, SHA-256, row classifications, suspect row
+    numbers and mapped source values for every recognized IFC object
+  - does not persist a draft, mutate project state or create a revision
+
+Parser errors expose stable codes such as `header-not-found`,
+`ambiguous-header`, `invalid-ifc-guid` and `duplicate-ifc-guid`. A later
+confirmed import must upload and parse the file again and verify its hash.