Per-target outbound seq counter on MeshState allocates a monotonic seq
before each typed envelope is encoded; send_typed_wire +
send_channel_typed_wire record it (alongside our own pubkey_hex) on the
Sent MeshMessage so the local store carries the same MessageKey the
receiver will see. TypedEnvelope.with_seq lets the RPC layer stamp the
seq AFTER signing (signature covers t/v/ts only).
New MessageKey struct pairs sender_pubkey+sender_seq as the stable
cross-transport identity. Adds variants 13 Reply and 14 Reaction with
ReplyPayload {target, text} and ReactionPayload {target, emoji}, plus
mesh.send-reply / mesh.send-reaction RPCs and receive-side dispatch
arms that store the payload json for the UI to index.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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