The core-owned transport-fault vocabulary: why a network operation could not deliver a response, reported as a value.

A client library reports a transport failure as its own exception type (http-client's HttpException, amazonka's error sum). Carrying those types through the agnostic tiers would couple every consumer to every client library, so the adapter edge -- the one place a library's exception type is already in scope -- classifies the failure into this closed vocabulary, and everything above it reasons over the value. The classification is deliberately coarse: it distinguishes only the causes a consumer or an operator reads differently (a timeout, an unreachable peer, a TLS refusal, or any other protocol-level fault); everything finer rides in tfDetail, rendered for a log line and never parsed.

This is a leaf module by design: the registry read path, the mirror queue, and the advisory sync all speak it, so it must sit below each of them.