Publish mixed-backend execution and readback contracts

[Brad-Edwards]

Expose executable bridge, time-coordination, and handoff obligations through portable contracts rather than descriptive topology references alone.

Acceptance criteria

• Publish versioned operation-owner bindings, action/subject mappings, bridge and coordinator interfaces, timing/order requirements, loss declarations, and readback evidence.
• Connect capability admission and contextual refusal to the shared operation protocol. Represent execution, delivery, observation, and partial/unknown results distinctly.
• Cover simultaneous mixed execution and staged handoff only within the accepted supported arrangements; reject missing mappings and unsupported coordination requirements.
• Update API-407 contracts, public backend interfaces, schemas, publication manifests, generated bundles, and migration examples consistently.

Prerequisites: #1355, #1360.

References: Diagnosis.