InMemoryExactTxPipelineStore

Counts admission rows that have not crossed into the application journal.

Implementations must perform one bounded primary-key scan rather than repeatedly decoding and sorting every record page.

Returns a page in implementation-specific order. SwayDB uses serialized key order; the in-memory store uses pipeline-id string order. Callers must not assume the same ordering across stores or snapshot isolation between pages while writes are in progress. Startup reconciliation runs before admission and is order-independent.