Commit several drafts of the tree store atomically

pure_draft_commit_all publishes drafts of one store with a single head:
every key must still be at its draft's base revision, all new extents and
catalog records are written and synced once, and either every document
becomes visible or none does. A single draft commit is the one-draft case.

pure_state_open stores runtime-owned values (app states) under '.' plus a
key. Document keys never start with '.', listings skip these values, and
document operations refuse their names, so an app's state cannot collide
with a document a client names.

Tests kill a process at every commit stage of a three-draft batch and see
all or none, inject each fault, stop a batch on one stale draft, and
commit fifty keys in no order at once.

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