On macOS, GuestWriteWatchCollectFresh reported nothing and returned true, so
MarkWritten took pages no collection had armed yet for clean, and they stayed
writable without being tracked. GuestWriteWatchCollectArmed was a full collect,
which also took those pages.
Pages now carry a fresh bit from registration until a collection takes them, as
the Linux watch keeps its fresh ranges: CollectFresh reports and arms only fresh
pages, CollectArmed skips them, and Collect takes both. Protection changes, host
writes and faults keep the bit.
The write_watch fresh and armed tests check both collections.
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