openai/gpt-chat-latest → B (9:1)

Side B makes lasting architectural changes: it introduces a RocksDB-backed `durable` workspace crate, adds an `EntityStore` to move raw Reddit JSON off-heap, changes reducer state to stop storing raw payloads in memory, and rewrites event log replay to stream events line-by-line instead of loading the entire log. Side A mainly adds a detailed migration plan document plus a thin `RouteContext` wrapper and re-export, which is useful guidance but contributes little functional behavior compared with B's concrete storage and startup-memory improvements.