Flow canvas: mark streaming edges with a marching dash

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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

frontend/src/components/Flow/flow.css

@@ -71,6 +71,32 @@
       stroke-width: var(--xy-edge-stroke-width);
     }
   }
+
+  /*
+   * An output that publishes repeatedly during one run reads as a march rather
+   * than a line. Only the dash offset moves, so the pulse above still shows
+   * through on the same path: it recolours and widens the dashes as they go.
+   * A loop rather than a one-shot, so none of the `--duration-*` tokens fit —
+   * the literal is one dash period per step, slow enough to read as direction.
+   */
+  .react-flow__edge-path.edge-stream {
+    stroke-dasharray: 6 4;
+    animation: edge-march 700ms linear infinite;
+  }
+
+  /* Both at once: one `animation` declaration has to carry both, or the
+     cascade keeps whichever rule came last and drops the other. */
+  .react-flow__edge-path.edge-stream.edge-live {
+    animation:
+      edge-pulse var(--duration-pulse) var(--ease-emphasized),
+      edge-march 700ms linear infinite;
+  }
+
+  @keyframes edge-march {
+    to {
+      stroke-dashoffset: -10;
+    }
+  }
 }
 
 /* A node that just published something says so, once, and settles. */