[Mirko-von-Leipzig]

Retains four recent tree generations in RecentSnapshots. Readers look up the generation matching their SQLite transaction’s tip, so they can acquire a consistent view even after newer trees have been published. Acquired snapshots remain valid after cache eviction.

This implements the snapshot-retention follow-up proposed in #2859 and reduces retries when the writer advances during read acquisition. The current write order still requires waiting when SQLite is ahead of tree publication. Retained generations keep tree caches and metadata alive and conservatively delay SQL history pruning; the capacity is fixed at four, including the latest generation.

Changelog

[[entry]]
scope       = "rpc"
impact      = "changed"
description = "Reduce read acquisition retries by retaining four recent tree snapshots."