Keep draft versions readable within a transaction

A module event may read a document and then edit it; the value it read must
not change. pure_draft_version captures an immutable version and
pure_draft_read_version lends its extents; pure_draft_hold suspends arena
reclamation while a transaction still reads earlier versions. A test reads
the original 200 KB after 400 held edits and checks reclamation on release.

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