Fieldbus integration interface

Gap capability: the shape by which fieldbus protocol stacks (EtherCAT, Modbus, Profinet, CIP) plug into the runtime — without committing to any specific protocol implementation in the core.

Feature: Fieldbus integration interface FEAT_0023
status: open
satisfies: FEAT_0010
is satisfied by: REQ_0120, REQ_0121

The shape by which fieldbus protocol stacks (EtherCAT, Modbus, Profinet, CIP) plug into the runtime — without committing to any specific protocol implementation in the core.
Requirement: Adapter-driven I/O REQ_0120
status: open
satisfies: FEAT_0023

The runtime shall expose an adapter trait by which a fieldbus driver produces Channel<T> / Subscriber<T> bindings for ingested process variables and consumes Publisher<T> for outputs.
Requirement: Out-of-tree driver crates REQ_0121
status: open
satisfies: FEAT_0023

Fieldbus driver implementations shall live in separate crates and shall not require modifications to the executor core.
Requirement: Protocol-neutral runtime REQ_0122
status: open
satisfies: FEAT_0010

The executor core shall not embed any specific fieldbus protocol implementation; protocol selection is a deployment concern carried in adapter crates.