userState key under which trailers received on a request are surfaced once the dispatch loop has reassembled the trailing HEADERS block.

userState key under which response trailers are stored on the HttpContext. Public so that callers driving HttpResult directly (the HTTP/2 writer in particular) can locate the recorded trailers without adding a struct field to HttpResult.

Get the response trailers previously recorded on the context, or an empty list if no trailers have been set. Order is preserved in the order they were added.

Get the request trailers (trailing HEADERS received after the DATA stream), or an empty list if no trailers were received. Populated by the HTTP/2 dispatch loop.

Set (or replace) a single response trailer. Analogous to `Writers.setHeader`: a later call with the same name (case-insensitive) supersedes the earlier one. Under HTTP/1.x this is a no-op semantically — the trailer list is recorded but never emitted.

Set the entire response trailers list, replacing anything previously recorded. The list is not validated here; the HTTP/2 writer will run `validate` before emitting and treat a failure as a stream-level PROTOCOL_ERROR.

Validate a list of trailer fields. Returns the first violation encountered, or `Ok ()` if all fields are permitted as trailers.

Validate a single trailer field. Returns `Ok ()` if the field is permitted as a trailer per RFC 7540 §8.1, or `Result.Error ProtocolError` otherwise. The malformed-trailer-block disposition is a stream-level PROTOCOL_ERROR (RFC 7540 §8.1.2 / §8.1.2.6).