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/adapter_secure_native.go

@@ -48,6 +48,7 @@ func (adapter *Adapter) applySecureReset() {
 	adapter.treeMutex.Lock()
 	adapter.tree, adapter.documentTree, adapter.documentTextTree = nil, nil, nil
 	adapter.treeMutex.Unlock()
+	adapter.clearStashedDocuments()
 	adapter.clearNativeDocument()
 	adapter.clearNativeReview()
 	adapter.clearMouseApplication()