Make web document navigation survive live-updating pages

Recreated-node caret recovery, tolerant caret validation, per-root retained document trees and cursors across tab switches, document warm-up, coalesced structural moves, and hung-provider fast-fail with an 'Application is not responding' announcement.

internal/atspi/document_error.go

@@ -38,13 +38,7 @@ func (adapter *Adapter) document_object_error(object ObjectID, err error) error
 		"could not be processed as object "+object.Path+" does not exist") {
 		return err
 	}
-	adapter.treeMutex.Lock()
-	trees := []*AccessibleTree{adapter.tree, adapter.documentTree, adapter.documentTextTree}
-	adapter.treeMutex.Unlock()
-	for _, tree := range trees {
-		if tree == nil {
-			continue
-		}
+	for _, tree := range adapter.ownedTrees() {
 		tree.mu.RLock()
 		entry := tree.nodes[object]
 		tree.mu.RUnlock()