refactor(sync): split SyncDelegate impl into focused submodules

Move the monolithic `nodedb/sync_delegate.rs` into a
`nodedb/sync_delegate/` directory with three focused modules:
- `mod.rs`: core SyncDelegate trait impl (producer state persistence,
  stream seq coordination, engine outbound drain/ack methods)
- `array_handlers.rs`: inbound array delta application
- `definition_apply.rs`: inbound collection definition application

The trait surface itself gains async drain/ack methods with explicit
durable-key tracking to support the delete-on-ack model introduced by
the durable outbound queues. Idempotent-producer state (producer_id,
accepted_epoch) is persisted to storage and reloaded on reconnect.

Author: farhan-syah