The package README (both languages) still described this layer as
protocol-only with the PythonCodeRuntime implementation deferred to a
later PR, contradicting the shipped code. Rewrite the intro to describe
the registered runtime, add a Configuration section for every Config cap,
and drop the "implementation not in this layer" limitation. Also pin the
residual-detach fixture's size invariant: the byteLength assertion only
holds above Node's Buffer pool threshold.