Presses during an in-flight page action or page switch were dropped
(the command was disabled while busy), so quick Undo presses were lost
and the replay test undo_walks_back_through_new_pages_and_their_titles
flaked under load. They now queue and are taken in order once nothing
is on its way. An action that shows a page counts as switching until
it lands.

Replay gains `settle`, and `accessibility` waits for the same settled
state before writing the tree, so replay tests stop racing background
work.

Assisted-by: claude-opus-5.5