HTTP2::Connection::FieldSection

Builds a copy of section with a validator's parse result attached. FieldSection is a struct that flows through the stream event channel, so the delivered event has to be built fresh here rather than mutated in place — every getter above is read-only, so there is nothing on the original to mutate anyway.

The connection-side ResponseValidator's already-parsed result for this section, attached (see the second initialize below) by Connection#transition_and_deliver once validation completes — parsed_response for a response's leading field section, parsed_trailers for its trailer section, never both. Both stay nil when no validator ran (server mode, or validation disabled) or for a PUSH_PROMISE section, which is never routed through the validator at all; the consumer (Client#await_response / its trailer counterpart) falls back to parsing fields itself in that case. Safe to read from a fiber other than the one that attached it: HTTPSemantics::ResponseSection is an immutable record, and the attached Headers is handed over only after HTTPSemantics.validate_trailers has finished building it and never mutated afterward — the validator that built it discards its own reference, and this struct exposes it via a read-only getter with no mutator, so there is no writer left to race the client's read.