#930 Approvals via Futex; local human_approvals kept as webhook-fed projection

EARS SPEC: - tools.approval.request shall keep its kwargs and return shape and create a Futex decision. - When Futex delivers decision.terminal (HMAC verified), the engine shall update the local projection, and the polling tool shall resume. - /approvals routes used by highway-mcp shall keep working. Parent epic: #913. Synthesis (technical problems / solution domains / alternatives) recorded as a comment when started.