Refined IPv6 packet — payload narrows by shape (TCP / UDP / ICMPv6 / ESP / raw bytes) and the host's nextHeader field is the on-wire discriminator.

@binstruct/ipv6 is a header-only (v0.0.1) coder: it does not walk extension headers (Hop-by-Hop, Routing, Fragment, Destination Options). A packet whose nextHeader names an extension header rather than a real upper-layer protocol lands in the raw fallback here too — that's a pre-existing @binstruct/ipv6 limitation, not something this layer can see through.