pub const ENCAPSULATION_HEADER_BYTES: usize = 4;Expand description
The encapsulation header the payload crossing our vtable carries: 2 bytes of representation id + 2 of options.
TOP LEVEL ONLY, not per nested struct — which is why the Cyclone backend
computes total = paylen + 4. size_bound deliberately does not include
it; max_serialized_size does.