Function warn_unusable_snapshot 

pub(crate) fn warn_unusable_snapshot(
    account: &str,
    provider: Provider,
    source: UsageSourceKind,
)

A cached snapshot that cannot be used under this backend.

Warns rather than falling back quietly: the redb row is rewritten only after a successful poll, so a stale one outlives a forge.toml edit and is re-seeded every boot. An account whose provider changed and whose new endpoint is failing would otherwise carry no figures indefinitely with nothing anywhere saying why.