GraphReFly storage tier dispatch + Node-side persistence.

Implements the G.27 storage tier protocol: tiered N-way storage with per-tier transactions, debouncing, compaction, and codec parameterization. Phase 13.6's deferred ACID atomicity tightening lands here via redb, which provides pure-Rust ACID transactions without a C dependency.

Status (M4.D — 2026-05-11)

• [wal] — WAL frame substrate + canonical-JSON SHA-256 checksum (DS-14-storage Q1 + Q5 locks).

• [error] — [StorageError] / [RestoreError] / [RestoreResult].

• [codec] — [Codec] trait + [JsonCodec] (canonical-JSON encoding, parity with TS jsonCodec).

• [backend] — [StorageBackend] trait + [MemoryBackend] + [memory_backend] factory.

• [tier] — [BaseStorageTier] + typed sub-traits ([SnapshotStorageTier], [AppendLogStorageTier], [KvStorageTier]).

• [memory] — concrete generic structs SnapshotStorage / AppendLogStorage / KvStorage + factories memory_snapshot / memory_append_log / memory_kv.

• [file] (feature file) — [FileBackend] + [file_backend] factory + atomic-rename write via tempfile + percent-encoded filenames (byte-identical to TS fileBackend).

• [redb] (feature redb-store) — [RedbBackend] + [redb_backend] factory + ACID per-write transactions via [redb::Database]. Structurally closes F1 (pending retry-safety at tier level) + F3 (concurrent flush race).

• [graph_integration] — Graph-level storage integration (M4.E2): attach_snapshot_storage, restore_snapshot, snapshot diff engine, GraphCheckpointRecord. Free functions (D170) because graphrefly-graph does not depend on this crate.