feat(block-editor): wire Linear, Polar and XY parameter placement

Enable the three existing palette rows and BPARAMETER choices. Pick a base,
endpoint and label location to create the appropriate native parameter, one
end grip and its location expressions in the shared authoring graph.

Render editor dimensions and labels, preserve them when reframing the block,
and include them in editor extents. Reuse local undo/redo, save/discard and
native DWG/DXF persistence. Reject incomplete or invalid placement without
changing document data or history.

This milestone implements parameter placement. Action association/evaluation,
parameter property editing, alternate grip counts and parameter sets remain
separate work.

Validation: focused placement and palette interaction checks; Block Editor and
dock regressions; native append and DWG/DXF round trips in all 17 research
fixtures, preserving existing graph data and source files; app-view renders.

src/app/command_driver/mod.rs

@@ -575,6 +575,9 @@ impl OpenCADStudio {
                     return task;
                 }
             }
+            result @ CmdResult::PlaceBlockDistanceParameter { .. } => {
+                return self.handle_place_block_distance_parameter(result);
+            }
             result @ CmdResult::PlaceBlockPointParameter { .. } => {
                 return self.handle_place_block_point_parameter(result);
             }