pdn: correct the instance obstruction calculation

Use max(halo, min_space) rather than halo + min_space to bloat the instance.
This was causing gf12/tinyRocket to fail due to false strap / obs overlap.

Signed-off-by: Matt Liberty <mliberty@precisioninno.com>

src/pdn/src/grid.cpp

@@ -1347,11 +1347,13 @@ ShapeTreeMap InstanceGrid::getInstanceObstructions(
   for (auto* ob : master->getObstructions()) {
     odb::Rect obs_rect = ob->getBox();
 
-    // add min spacing
+    // add max of min spacing and the halo
     auto* layer = ob->getTechLayer();
-    obs_rect.bloat(layer->getSpacing(), obs_rect);
+    odb::Rect spacing_rect;
+    obs_rect.bloat(layer->getSpacing(), spacing_rect);
     obs_rect = applyHalo(obs_rect, halo, true, true, true);
-
+    obs_rect.merge(spacing_rect);
+
     transform.apply(obs_rect);
     auto shape = std::make_shared<Shape>(layer, obs_rect, Shape::BLOCK_OBS);