WIP: RemoteResource MVP prototype

[kd-zero]

Summary

• add Fulfil::RemoteResource as a Fulfil-specific domain resource base class
• add Fulfil::RemoteProduct as the first concrete subclass
• wire new classes via lib/fulfil.rb
• add unit tests for base behavior and product mapping
• add RDoc comments for the new remote resource APIs
• document remote resource usage in README

Current shape

Subclasses are expected to define:

• FULFIL_MODEL_NAME
• ATTRIBUTES
• .from_fulfil(response)

Notes

• intentionally Fulfil-specific (no adapter abstraction)
• intentionally minimal MVP; create/update/pagination can be added next