Action delivery

Use ActionDeliveryGate between Relay output and the controller.

Input	Gate behavior
New generation	Accept or replace the active chunk
Old generation/sequence	Reject as stale
Different session/model	Reject
Late chunk or old observation	Reject as expired
Unsafe value	Reject the chunk or clamp each control step
Missed control tick	Skip obsolete steps by timestamp

auto gate = fe::relay::ActionDeliveryGate::create(
    model,
    {.control_dim = 7,
     .overlap_steps = 2,
     .step_period_ns = 10'000'000,
     .max_source_age_ns = 80'000'000,
     .unsafe_policy = fe::relay::UnsafeActionPolicy::kClamp},
    {.lower = lower, .upper = upper, .max_delta_per_step = max_delta});

gate->accept(action_message, now_ns);
if (auto step = gate->next(now_ns))
  controller.write(step->values);

action_dim is the flattened chunk size: step_count × control_dim. Setup copies limits once; accept, replacement, overlap, and next allocate nothing. Run action_delivery_sample for a 25 Hz policy feeding a 100 Hz controller.

This is a deterministic software gate, not a certified robot safety controller. Hardware limits, watchdogs, and emergency stops remain downstream.