Allow drafts without a stored base

Runtime-owned values such as an app's text need the same extent tree as
stored documents without a store file. pure_draft_new creates such a draft;
it edits, lends versions and reclaims like any draft but refuses to commit.
pure_draft_restore returns a held draft to an earlier version, so a failed
event can abandon its edits. A test runs 4,000 random edits with a fifth
abandoned against a flat oracle under ASan/UBSan.

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