Record the messenger between two nodes and its latency (089)

The roadmap's E2EE messenger made of the runtime's general parts, with
no messenger code in it: sealed, authenticated sessions between nodes
(086, 088), Vega on each node's display as a surface (design 010), the
app in Vega's examples. Vega's check runs two nodes through a recording
relay: a line typed on one display is drawn on the other, frames match
the vega command's, and the relay sees no text.

A line takes 0.79 ms at p50 and 1.1 ms at p99 from one node's program
to the other node's views with stores in RAM, both commits, the HTTP
delivery and the seal included (sealing adds nothing measurable); on the
Pi's SD card 3.5 ms at p50, the long tail being the card's syncs.

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