Scope: decode is always reachable; encode depends on how the adapter was built. The flat ABI used by third-party codecs is input-bytes → output-bytes, which doesn't expose the structured `Encoded` return shape the in-tree trial-encode loop expects. An adapter built through [`ThirdPartyPlaneCodec::new`] represents a codec the trial-encode loop is considering among several candidates, so `encode` returns `Err(InvalidContainer)` there: reaching `encode` on that path would mean the trial loop reached a third-party codec, which is itself a bug. An adapter built through [`ThirdPartyPlaneCodec::new_explicit`] represents a codec the caller named directly; there is no trial loop to satisfy, so `encode` is reachable.