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/review/document_benchmark_test.go

@@ -37,7 +37,7 @@ func BenchmarkDocumentWorkerHeadingNavigation(b *testing.B) {
 			b.ReportAllocs()
 			for b.Loop() {
 				for _, backwards := range []bool{false, true} {
-					result, next := worker.moveDocument(ctx, cursor, DocumentHeading, 2, backwards, 0)
+					result, next := worker.moveDocument(ctx, cursor, DocumentHeading, 2, backwards, 0, 0)
 					cursor = next
 					want := ids[middle+20]
 					if backwards {