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_caret_line.go

@@ -42,10 +42,27 @@ func move_caret_object_line(ctx context.Context, reader DocumentCaretReader, doc
 		offset = indices[position].end - 1
 	}
 	initialPosition := position
+	// Structural navigation already announced its target's whole line,
+	// including embedded children such as a heading's link. A following
+	// Down must leave that subtree, not descend into it and repeat the
+	// announcement as a new line. The target's own first range still uses
+	// the same-current logic below, so a long structural paragraph keeps
+	// advancing line by line within its own text.
+	structuralEnd := -1
+	if place.origin == StructuralNavigation {
+		structuralEnd = doc.SubtreeEnd(current)
+	}
 	for attempts := 0; attempts < len(indices)+1; attempts++ {
 		if position < 0 || position >= len(indices) {
 			return DocumentCaretMove{Position: documentPosition(doc, *place)}, nil
 		}
+		if command == CaretNextLine && structuralEnd >= current &&
+			indices[position].index >= current && indices[position].index <= structuralEnd &&
+			!(position == initialPosition && indices[position].index == current) {
+			position++
+			offset = 0
+			continue
+		}
 		node := doc.Nodes[indices[position].index]
 		if named_link_label(node.Snapshot) != "" && (command == CaretLineStart || command == CaretLineEnd) {
 			offset = 0