Verify executable mixed-backend capability claims #1017

Backend conformance must establish that declared mapping, timing, handoff, and readback capabilities are executable.

Acceptance criteria

• Probe the selected allocation, mappings, bridge operations, time coordination, ordering, handoff, and declared losses through the revised contracts and shared runtime.
• Reject missing/incorrect mappings, unsupported time guarantees, stale handoffs, and claims supported only by a common interface or metadata.
• Check repeated mixed-control transitions and backend refusal at the actual dispatch boundary. Preserve execution, delivery, observation, and partial/unknown outcome distinctions.
• Publish independently runnable positive/negative cases with exact backend/configuration and evidence provenance. Report unsupported combinations explicitly; a reference fixture is not concrete backend realization.

Prerequisites: #1372, #1364.

References: Diagnosis.