Define cross-store ordering semantics for TierStore listings #1039

Noting here that #692's TierStore::list and TierStore::list_paginated currently return only keys from the primary store. This is intentional until listings from the primary and ephemeral stores can be merged correctly. Merging is currently not possible because PaginatedKVStore tokens are opaque and store-specific.

Although individual stores may order entries by creation time, TierStore cannot:

• Compare the creation order of entries across stores.
• Interleave entries in the expected newest-first order.
• Construct a portable continuation token representing progress in both stores.

What is needed is a way to define ordering and pagination semantics for a combined listing before exposing keys from both stores.