dashboard: messages follow the line they went over

Dots and flashes ran straight from sender to receiver, whatever line
the message took: a PING over another transport, drawn as a curved
line beside the one in use, crossed the map off its line.

Each message now runs along the line it went over, as drawn (`route`:
the link's quadratic curve, its control point where renderGraph put
it): a message naming the address it went to (the paths layer's PINGs
and PONGs) over that address's transport's line, any other over the
line in use; a flash's streak bends with it.

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