Fix #784: validate selection IDs and report the resulting selection

docs/mcp.md

@@ -612,6 +612,15 @@ the one highlighted layer or group row, else the one selected group, else the cu
 so art added later is clipped too); called again it releases the mask. It returns `{clip}`, and the Layers panel
 underlines clipping-path names.
 
+## Object selection
+
+`select.set {ids}` replaces the selection, `select.add {ids}` adds to it, and
+`select.toggle {id}` or `select.toggle {ids}` toggles each target.
+Every id must be a non-negative integer naming an existing object.
+A malformed or unknown id is an error naming that value; the entire selection is left unchanged.
+All three commands return `{count, ids}` for the resulting selection, in selection order.
+An empty array clears the selection for `select.set` and leaves it alone for Add and Toggle.
+
 ## Saved selections
 
 Select → Save Selection… keeps the selected objects under a name, in the document: `select.save {name?}` (default