RESP command dispatch boundary for FerricFlow commands.

The Rust RESP parser builds Flow AST tuples and this module forwards them to the public FerricStore.flow_* APIs, normalizing replies into Redis-friendly values. It intentionally stays thin so command parsing, API validation, and Raft apply logic remain separate.

Performance boundary

Flow write commands can be batched by the connection pipeline before reaching here. Keep this dispatcher mechanical: no behaviours/protocols, no per-command dynamic lookup tables in hot writes, and no response hydration beyond API contracts.