Chat bots need replies, threads and reactions. Add two optional, omitempty
envelope fields (ThreadID, ReplyTo) plus a REACT frame type. The fields ride the
cleartext envelope (message-id references, not secret content) and are omitted
when unset, so non-threaded frames are byte-for-byte identical on the wire and
their signatures unchanged — a non-breaking, additive change.
Client gains PublishReply (threaded reply) and React (emoji reaction). The
reaction content travels in the payload, so it is sealed like any message and
stays confidential in E2E rooms; receivers dispatch on Frame.Type == REACT and
read Frame.ReplyTo for the target. Publish is refactored to share one
publishFrame path with the new helpers; its behavior is unchanged.
Tests: frame round-trip of a threaded REACT frame (golden), non-threaded
wire/sig back-compat asserting thr/re keys are absent (edge), Unmarshal of
garbage errors (error path), and an end-to-end reply+reaction round-trip in an
encrypted ModeMatrix room.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
egutierrez
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