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/worker_contract.go

@@ -112,6 +112,7 @@ type Result struct {
 // activation, fragment, flat unit, then object direction. Flags are deliberately
 // retained; epoch and timeout retirement depend on their distinct combinations.
 type request struct {
+	warmDocument      bool
 	currentLine       bool
 	currentLineBrowse bool
 	documentList      *documentListRequest
@@ -126,6 +127,7 @@ type request struct {
 	readTree          *atspi.AccessibleTree
 	readFocus         atspi.ObjectID
 	documentEpoch     uint64
+	documentLoadEpoch uint64
 	queuedAt          time.Time
 	target            DocumentTarget
 	caretCommand      DocumentCaretCommand