Execute mixed-backend mappings, coordination, and handoffs

Mixed dispatch must execute admitted mappings and time obligations and derive outcomes from backend readback.

Acceptance criteria

Invoke the admitted mapping/bridge and time-coordination interfaces before dispatching to the owning backend. Preserve subject, operation, configuration, and ordering identity.
Execute supported handoffs with their required state and authority checks; refuse stale, unmapped, unsupported, or contextually rejected work without inventing completion.
Record execution, delivery, and observation only from their corresponding evidence. A backend success boolean cannot establish participant observation.
Verify a non-identity mapping, time-coordination failure, contextual refusal, partial execution, stale handoff, and missing delivery evidence through the shared runtime.

Prerequisites: #1371, #1362.