Demonstrate mixed control using executed mappings and backend evidence #1018

Produce a reproducible demonstration of the repaired shared-runtime control and mixed-execution paths against explicitly selected backend realizations.

Acceptance criteria

• Pin a supported capability matrix and executing backend artifacts; add native blockers for any unfinished backend realization they require. Include simultaneous mixed execution and a staged handoff; separate single-backend runs do not establish mixed support.
• Exercise repeated control changes, a governed effect, a non-identity mapping, and time/order coordination. Include denial, refusal, stale handoff, and missing/failed delivery.
• Compare runtime outcomes with backend realization/readback and show that prohibited effects did not occur. Preserve participant, episode, operation, mapping, timing, policy, and configuration identity.
• Publish the runnable scenario, commands, evidence, and independent reproduction record for the selected configurations. Do not claim universal backend equivalence or simulation-to-operational transfer.

Prerequisites: #1071, #1017.

References: Diagnosis.