A port with `stream` set publishes repeatedly during one execution, and
until now nothing on the canvas said so. `deriveEdges` carries the flag
from the producing `MessageSpec` onto the edge, `LiveEdge` turns it into
an `edge-stream` class, and the class draws a dashed stroke whose offset
marches toward the target. The pulse still lands on top: both animations
share one declaration when a value arrives on a streaming edge.
stroblme
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