Implement tiered storage

This commit adds `TierStore`, a tiered `KVStore` implementation that
routes node persistence across three storage roles:

- a primary store for durable, authoritative data
- an optional backup store for a second durable copy of primary-backed data
- an optional ephemeral store for rebuildable cached data such as the
  network graph and scorer

TierStore routes ephemeral cache data to the ephemeral store when
configured, while durable data remains primary+backup. Reads and lists
do not consult the backup store during normal operation.

For primary+backup writes and removals, this implementation treats the
backup store as part of the persistence success path rather than as a
best-effort background mirror. Earlier designs used asynchronous backup
queueing to avoid blocking the primary path, but that weakens the
durability contract by allowing primary success to be reported before
backup persistence has completed. TierStore now issues primary and backup
operations together and only returns success once both complete.

This gives callers a clearer persistence guarantee when a backup store is
configured: acknowledged primary+backup mutations have been attempted
against both durable stores. The tradeoff is that dual-store operations
are not atomic across stores, so an error may still be returned after one
store has already been updated.

Additionally, adds unit coverage for the current contract, including:
- basic read/write/remove/list persistence
- routing of ephemeral data away from the primary store
- backup participation in the foreground success path for writes and removals

Note: We skip the Rust-only backup-store builder test when UniFFI
bindings are enabled, since backup and ephemeral store configurations
are not exposed through FFI yet (wiil be added in #871 follow-up).

Author: enigbe

src/io/mod.rs

@@ -12,6 +12,7 @@ pub mod postgres_store;
 pub mod sqlite_store;
 #[cfg(test)]
 pub(crate) mod test_utils;
+pub(crate) mod tier_store;
 pub(crate) mod utils;
 pub mod vss_store;