Get ledger statistics

Returns aggregate usage statistics (transaction, volume, reference, posting, log, revert and Numscript-execution counts, plus ephemeral-evicted and transient-used volume tallies) for a ledger. Requires ledger:read scope. On live reads, transactionCount and logCount are FSM-backed. The seven usage-derived counters (postingCount, revertCount, numscriptExecutionCount, referenceCount, ephemeralEvictedCount, transientUsedCount, volumeCount) are materialised asynchronously in the serving replica's usagestore and may lag committed state even after the request's ReadIndex barrier. Note: for checkpoint-scoped (historical) reads, only transactionCount and logCount are checkpoint-consistent; the usage-derived counters (postingCount, revertCount, numscriptExecutionCount, referenceCount, ephemeralEvictedCount, transientUsedCount, volumeCount) are sourced from the live usagestore projection, which is not checkpoint-aware, and are returned as 0 at a checkpoint rather than a live value — treat 0 on a checkpoint read as "not available at this checkpoint".