Single-version storage backend for workloads where snapshot isolation would only add overhead. Implements the
SingleVersionStore family of traits from core::interface::store: get, contains, set, remove, commit, and the
ranged scan iterators used by the engine for table and index walks.
Writes are atomic per commit but never coexist with prior versions of the same key; readers always observe the latest committed value. The buffered tier batches recent writes and the persistent tier owns durable state, the same shape as the multi-version backend minus history.
Invariant: a key's value after commit is the value the next reader sees - no version cursor, no time travel. Code
that needs history must use store-multi; reaching here for it returns nothing useful.